The Archive of the DAp Seminar Video Recordings
WARNING: to access the links to video streams and PDF slides below, you need to have a CEA Intra account.
This page provides links to the video recordings of most of the DAp seminars since 2021, together with their accompanying slides, when available. The video recordings and the PDF slides are stored on the eMedia platform of CEA, which requires being on Intra and having a Mobipass. If you do not have a Mobipass, ask Hugues DIAMBOTE (room 156).
To find seminars, click on any of the search buttons below. A list of seminars will appear, showing the date, speaker and title.
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Tuesday January 9 2024 | Roya MOHAYAEE (IAP)
The Anomalous Dipole Summary [click here] Standard model of cosmology is based on the cosmological principle, which states that the Universe is statistically homogeneous and isotropic on large scales. Is this hypothesis supported by the observations ? After a historical survey of the field, I shall use the high redshift data from
radio galaxies and quasars to show that the early Universe does not seem to be isotropic and the rest frame of cosmic microwave background radiation does not coincide with the rest frame of distant sources. I shall also demonstrate that the cosmological principle is violated at a statistical significance of over 5-sigma. |
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Tuesday January 23 2024 | Francisca KEMPER (Institute of Space Sciences (ICE-CSIC) / ICREA / IEEC, Spain)
The interstellar dust reservoir in galaxies Summary [click here] The evolution of interstellar dust reservoirs, and the evolution of galaxies themselves go hand-in-hand, as the presence of dust alters evolutionary drivers, such as the interstellar radiation field and the star formation history, while at the same time, the dust is being formed and altered by processes taking place in galaxies. However, far-infrared and submillimeter studies have revealed enormous dust masses at high redshifts that are difficult to explain with dust production from evolved stars (the so-called "dust budget problem"), while in the nearby universe there is also a significant mismatch between the dust production rate and the dust mass observed in the interstellar medium of galaxies. I will go over some possible explanations in an attempt to find a way forward towards a solution to this seeming discrepancy. |
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Tuesday January 30 2024 | Florian PACAUD (University of Bonn)
Galaxy clusters and filaments in the first eROSITA all-Sky survey Summary [click here] Launched in July 2019, eROSITA was set to survey the sky in the soft X-ray band for four years and provide a much awaited update to the old ROSAT all-sky survey. In a few weeks, the collaboration will release the first all-sky map obtained from the first six month of observation. In this talk, I will present the instrument and its technical abilities. I will then explore the all-sky map and show selected results on individual local clusters and filaments. Finally, I will give a preview of the soon-to-be-released data products, including AGN and galaxy catalogs, and present some of the methods used for their cosmological analysis. |
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Tuesday February 6 2024 | Stefanie WALCH-GASSNER (University of Köln)
The impact of stellar feedback on different scales and galactic environments Summary [click here] Stellar feedback refers to the processes by which massive stars release energy, radiation and material into their surroundings, influencing the structure and evolution of the galaxies in which they reside. Understanding the impact of stellar feedback on different galactic environments is crucial for developing a comprehensive picture of galaxy formation and evolution. In this context, different galactic environments refer to regions within a galaxy that differ in their physical conditions, such as the average gas density, temperature, or metallicity.
We study the respective impact of stellar winds, ionizing radiation, and supernovae in modern simulations of the multi-phase interstellar medium in parts of galaxies within the SILCC project, which I will present in this talk. From these galactic scale simulations we find that ionizing radiation is the most important factor in regulating the star formation rate, while supernova over-pressure the gas substantially, thus driving a galactic outflow. |
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Tuesday February 13 2024 | Jonathan TAN (Chalmers & University of Virginia)
A Light in the Dark - Massive Star Birth Through Cosmic Time Summary [click here] Massive stars are important thoughout the universe, but their formation remains poorly understood. I review current understanding of how massive stars and star clusters form in our Galaxy, including models for how star formation is triggered in giant molecular clouds and tests of how individual massive stars form from smaller scale clumps and cores. Finally, I discuss how massive star formation may have been different in the very early universe and how the first stars may have seeded the supermassive black holes powering active galactic nuclei. |
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Tuesday February 27 2024 | Hakim ATEK (IAP)
Unveiling the faintest and the brightest galaxies at early times with JWST Summary [click here] The JWST is revolutionizing our understanding of the early Universe by unveiling a wealth of bright galaxies at z>9 and faint AGNs at z>5. I will present the latest constraints on the overabundance of UV-bright galaxies at z>9, which is 10-100 times higher than galaxy formation models. I will discuss to what extent recent theoretical efforts can reproduce such observations, and how future wide-area surveys such as Euclid will help put stronger constraints ion the bright-end of UVLF at z>8. On the other hand, faint galaxies, representing the building blocks of present-day galaxies, have eluded spectroscopic constraints, even with the deepest JWST campaigns so far. I will present the results of our UNCOVER survey, which combines ultra-deep NIRSpec spectroscopy with the strong lensing magnification of A2744 cluster. We characterize ultra-faint galaxies with intrinsic absolute magnitude between Muv=-17 and Muv=-15 at 6<z<8, and stellar masses down to 10^6 solar masses. I will discuss our plans to obtain the deepest observations on sky with the GLIMPSE program to the faintest galaxy population out to z=15 and beyond. |
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Tuesday March 5 2024 | Recent hiree seminar Emmanuel BERTIN (LCEG)
Wide-field imaging meets deep learning: new challenges, new opportunities Summary [click here] Since the 19th century, wide-field imaging has significantly advanced numerous fields of Astrophysics, spanning from the study of solar system bodies to observational cosmology. Ongoing and future optical/near-infrared imaging surveys face many interesting data analysis challenges, especially in time domain astronomy, in a context where detector technology enable wide-field observations at increasingly high frame rates. Through concrete examples, I will show with how Deep Learning techniques offer promising solutions to address such challenges, and provide new scientific opportunities. |
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Tuesday March 12 2024 | Postdoc seminar Arturo NUNEZ (LMPA)
The impact of baryonic physics in galaxy formation: Multi-scale approach to galaxy simulations, from the initial mass function to galaxies in a cosmological environment Summary [click here] The interplay of baryonic physics (star formation and feedback processes) in shaping galaxies and their host halos is a multiscale and multidisciplinary problem. Numerical simulations are key to understanding these processes as they cover phenomena that take place at a wide range of scales, some too small for most astrophysical observations but yet able to impact the shape and evolution of galaxies as a whole. However, no single simulation can address every aspect of this complex issue. A comprehensive approach is essential to integrate learnings from various types of simulations and compare them with observations. This includes understanding star formation histories in cosmological simulations, examining interstellar medium dynamics in comparison to simulations of individual galaxies, and exploring the non-universality in the stellar initial mass function in specific molecular cloud simulations. My goal is to discuss how simulations across different scales, together with detailed multiscale observations, collectively contribute to approaching the complexities of galaxy formation and evolution. All this while discussing current shortcomings and successes of high-resolution numerical simulations of galaxies and galactic environments.
WARNING: incomplete video recording. |
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Tuesday March 19 2024 | Postdoc seminar Miguel FERNANDES MOITA (LISIS)
The Large Italian X-ray facility (LARIX) and the advancements in Laue lens development Summary [click here] The LARIX (LARge Italian X-ray facility) is a multi-project facility situated in the Scientific-Technological Pole of the University of Ferrara, housed in an underground building featuring a 100-meter-long tunnel with two large experimental rooms on each side. Dedicated to the development and testing of X- and Gamma-ray astronomy instrumentation, LARIX hosts two beamlines: the 12-meter-long LARIX-A in experimental room A, suitable for linearity tests of hard X-ray detectors, reflectivity measurements of X-ray reflector samples, and ground calibrations; and the 50-meter-long LARIX-T installed in the tunnel, ideal for testing gamma-ray reflectors and low-weight gamma-ray detector prototypes when requiring low-divergence beams or lengthy beamlines. This presentation will provide an overview of both installations, their instrumentation, past projects, and opportunities for access through collaborations or transnational access programs like AHEAD. Furthermore, we will discuss recent advancements in Laue lens development that we did in LARIX, particularly the TRILL project, supported by ASI, aimed at advancing the technological readiness of Laue lenses. Future goals include the ASTENA mission concept, submitted to the ESA program 'Voyage 2050', featuring a narrow field telescope (NFT) based on a focusing Laue lens with an energy passband from 50 to 700 keV and a 20-meter focal length, promising breakthroughs in sensitivity and angular resolution in this energy band. Additionally, we will explore innovative concepts such as a high-energy Laue lens spectro-polarimeter, inspired by the recent success of the IXPE mission. |
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Tuesday April 2 2024 | James DUNLOP (University of Edinburgh)
Charting the emergence of the first galaxies with JWST Summary [click here] The James Webb Space Telescope (JWST) is transforming our view of galaxy formation and evolution in the young Universe. I will provide an overview of the latest results from the PRIMER survey, the largest JWST Cycle-1 “Galaxies” programme which, in combination with other public JWST imaging, is now enabling us to chart the early growth of the galaxy population back to within ~300 million years of the Big Bang. Specifically, I will present and discuss the first robust determination of the evolving galaxy luminosity function extending out to redshifts z~13, as well as new measurements of the galaxy stellar mass function reaching out to z~9. I will then attempt to interpret these results in the context of our current understanding of the evolving dark matter halo mass function and the efficiency with which galaxies are able to convert their baryons into stars. Finally, I will discuss the prospects for future progress with JWST and with the Atacama Large Millimeter Array (ALMA), including the possibility of uncovering the emergence of the first galaxies. |
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Tuesday April 23 2024 | Florent MERTENS (Observatoire de Paris)
Unveiling the Physics of the Cosmic Dawn and the Epoch of Reionisation in the SKA era Summary [click here] Direct observation of the Cosmic Dawn and Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are underway aiming at statistically detecting the 21-cm signal fluctuations from the EoR and CD, and paving the way for the SKA EoR CD experiment which will be capable of directly image the large-scale neutral hydrogen structures from these distance epochs. In this talk, I will present recent results from the LOFAR-EoR and NenuFAR Cosmic Dawn collaborations. After publishing the deepest upper limit on the signal power-spectra at z~9, which has made it possible to set constraints on the physics of the IGM during the EoR, the LOFAR-EoR team is progressing towards a deeper upper limit on a broader range of redshift. On the Cosmic Dawn front, the NenuFAR CD team recenty published a first upper limit on the signal power-spectra at z~20. These new results will be discussed, as well as the improvements in instrument calibration and foreground mitigation that enabled these achievements. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Tuesday April 30 2024 | Postdoc seminar Dinil PALAKKATHARAPPIL (LDE3)
Precise Stellar Age Determination of Stars with Gaia and TESS: Prospects and Insights for the upcoming PLATO and HAYDN Missions Summary [click here] Determining precise star ages is essential for insights into stellar, exoplanetary, and galactic evolution, but it remains a complex challenge. The two most precise techniques are (1) asteroseismology, which is the study of the internal structure of stars by means of intrinsic global oscillations, and (2) analysis of stellar clusters where stars are formed from the same molecular cloud at the same time, and the observed colour-magnitude
diagram is a strong diagnostic of the cluster age. In this talk, I will combine these two techniques to determine the age of intermediate age open cluster NGC 2477. We used astrometry and colour from Gaia to extract membership probabilities and colour information about the stars in clusters, while pulsating stars were found using TESS data. Observations are complemented with asteroseismic modelling using MESA and GYRE to obtain the precise age of the cluster. In the second part of the talk, I will detail my contribution to the development of the ground segment of the ESA M3 PLATO mission to obtain stellar mass, radius, and age as well as several rotation and magnetic activity indicators for which CEA is responsible. I will conclude by showcasing some applications of the PLATO pipeline using Kepler and TESS data. |
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Tuesday May 7 2024 | Thomas HENNING (Max Planck Institute for Astronomy, Heidelberg)
Towards New Horizons: Star and Planet Formation with JWST Summary [click here] The James Webb Space Telescope (JWST) is the most powerful space-based infrared observatory ever built. The first part of the talk will discuss the main structures of the telescope and its assembly after launch. The talk will then highlight the extremely exciting science results in the field of star and planet formation, including the nature of protostars and planet-forming disks. A special emphasis will be placed on the accretion process and the chemical building blocks of disks and what we can learn from infrared spectroscopy regarding the inventory of material in the terrestrial planet-forming zone. |
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Tuesday May 14 2024 | Recent hiree seminar Damien TURPIN (DAp)
The high-energy and multi-messenger transient sky in the eyes of the SVOM mission Summary [click here] Over the past decades, the study of highly energetic transient phenomena has led to several scientific breakthroughs from the discovery of r-process nucleosynthesis production sites to the first credible identification of ultra-high-energy cosmic-ray accelerators. In a general manner, these explosive events allow us to study extreme physical regimes (extreme gravity, extreme magnetic fields, relativistic shocks) and are unique probes to study the Universe properties up to high redshift. In the last 10 years, an unprecedented amount of new generation instruments observing the sky at all wavelengths and with all astrophysical messengers have significantly contributed to enlarging our vision of the transient sky phenomena. This observational revolution initiated by the gravitational wave interferometers has forced the astrophysical community to rapidly adapt to the new challenges of multi-messenger astronomy. In June 2024, the French community will have a leading role in this young research domain with a strong implication for the French lab. in the upcoming SVOM mission. In this seminar, I will review some of the latest results obtained by the scientific community exploring the high-energy transient sky, especially with a focus on the sources of interest for the SVOM mission. I will show how important will be the SVOM mission contribution to this exciting field of research that has probably revealed only a subset of its richness. |
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Tuesday May 21 2024 | Climate seminar Céline GUIVARCH (École des Ponts ParisTech)
Le changement climatique vu à travers les lunettes d’une économiste Summary [click here] Les effets du changement climatique constituent une menace grandissante pour nos sociétés et nos économies; et agir face à cette menace nécessite de transformer nos façons de nous déplacer, nous loger, de produire et consommer. Autant de questions pour l'économie, pour analyser les implications économiques des trajectoires et politiques publiques d'atténuation du changement climatique. Ce séminaire proposera quelques incursions dans la recherche actuelle en économie du changement climatique, à partir d'un échantillon d'articles récents.
Le séminaire sera en français avec des slides en anglais. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday June 11 2024 | Mustapha MEFTAH (LATMOS)
Les NanoSatellites au service de l’observation de la Terre et de l’univers Summary [click here] Le thème de la conférence portera sur l’observation de la Terre, le climat, les satellites et le NewSpace. Le 24 janvier 2021, une fusée Falcon 9 était lancée avec succès par SpaceX depuis Cap Canaveral, en Floride. Sous la coiffe du lanceur, il y avait 143 satellites. C’est à ce jour le plus grand nombre de satellites jamais déployés en une seule mission. Uvsq-Sat, premier nanosatellite français dédié à l’observation de variables climatiques essentielles, faisait partie de cette épopée. Depuis plus de trois ans, Uvsq-Sat observe la Terre et le Soleil de manière assidue. A 550 km d’altitude, ce satellite mesure la quantité d'énergie solaire réfléchie vers l'espace et la quantité d'énergie thermique émise par la Terre vers l’espace.
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Tuesday June 25 2024 | Jérôme RODRIGUEZ, Bertrand CORDIER & Damien TURPIN (DAp)
The SVOM Launch: En route for the transient hunt Summary [click here] The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a Sino-French mission planned to be launched on June 22nd 2024 from Xichang China. The launch of the satellite opens the time when scientists will step up and start exploiting the data. SVOM, however, is already a long-term project during which the involvement of engineers, developers, and scientists has already been huge. In this presentation I will overview some of the aspects of the mission, the scientific programs and aims of the project, but also recall its history and try and acknowledge the involvement of many colleagues which permitted the achievement of this fantastic machine. I will also introduce some of the novelties and specificities of the mission, in particular all the aspects dedicated to the time domain astronomy and real time analysis, implying the development of a dedicated science center, specific tools, and shifts for a certain of us as burst advocates. Finally we will (try to) have a zoom with those of us that have attended the launch in China, so that they can share with us their feeling of the launch and first impressions post launch, in this opening scientific exploitation era.
The talk will be given in French with slides in English. |
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Tuesday September 10 2024 | Postdoc seminar Maxime LOMBART (LFEMI)
How to treat dust coagulation/fragmentation in 3D hydrodynamic simulations ? Summary [click here] Particles coagulation and fragmentation are ubiquitous (raindrop formation, air pollution, combustion, polymerization, astrophysics) and mathematically described by the Smoluchowski coagulation and the fragmentation equations. Several processes such as gas/particles dynamics, chemical reaction and radiative transfer depends on the evolution of the particle size distribution governed by coagulation/fragmentation. Tracking the size evolution of particles in 3D simulation is key for understanding, for instance, cloud formation and planet formation. Therefore, these equations must be accurately solved while preserving computational costs, which is a tremendous numerical challenge. However, current algorithms for solving coagulation/fragmentation suffer from an over-diffusion in the conditions of 3D simulations. To tackle this challenge, we developed a discontinuous Galerkin scheme to efficiently solve the conservative form of the coagulation/fragmentation equation. In particular, we aim to perform the first 3D simulations of dusty protoplanetary discs and protostellar collapse that include realistic coagulation/fragmentation. |
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Tuesday September 17 2024 | Postdoc seminar Arghyadeep PAUL (LDE3)
Star-Planet Magnetic Interactions: From Solar System to Exoplanets Summary [click here] The interaction between stellar winds and planetary magnetospheres has been a focus of research for decades. Within the heliospheric context, star-planet interactions similar to that between the Sun and the Earth reshapes the planet’s local magnetic environment leading to the formation of magnetospheres. Magnetic reconnection within these magnetospheres gives rise to helical magnetic flux ropes known as flux transfer events (FTEs). We developed a two-way coupled magnetosphere-ionosphere model to investigate the impact of FTEs on the planet’s ionosphere. For a specific case of an Earth like planet, the field aligned currents generated by these FTEs closely resemble observations of discrete dayside auroral arcs, suggesting FTEs to be a probable cause. In the context of exoplanetary systems, star-planet interactions vary depending on the planet's orbital location. For close-in orbits located in the sub-Alfvénic stellar wind, the Poynting flux generated by star-planet interactions can propagate toward the star, giving rise to stellar chromospheric hotspots. Our current work characterises and quantifies the efficiency of this energy transfer
between the planet and the star, revealing that a significant portion of the energy never reaches the star and is reflected by the stellar transition region back toward the planet. The firm detection and characterisation of such magnetic interactions from observations of chromospheric hotspots would also lead to constraints on the amplitude of the magnetic field of exoplanets, to which we are blind so far. Future research, utilising the previously developed magnetosphere-ionosphere model, will also explore how the presence or absence of a planetary ionosphere influences the Poynting flux generated by the planet. |
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Tuesday October 1 2024 | Recent hiree seminar Elsa DUCROT (LDE3)
Detection and characterization of rocky planets around ultra-cool stars Summary [click here] The launch of the James Webb Space Telescope (JWST) marked a significant milestone in the field of exoplanet research. For the first time, we can study the atmospheres of Earth-sized rocky planets—an opportunity that was previously unattainable. However, this is currently possible only around the coolest stars, known as red dwarfs. These stars are ideal targets for several reasons: (1) their smaller size makes it easier to detect and study small, transiting planets; (2) their lower luminosity leads to more frequent planetary transits for the same level of stellar irradiation; (3) they are the most abundant stars in the galaxy, and (4) planets orbiting them are more likely to be rocky planets with short orbital periods. In the near future, only such planetary systems are expected to produce signal-to-noise ratios (SNRs) high enough to confidently confirm or rule out the presence of atmospheres on potentially habitable planets.
In this context, my research focuses on: (1) detecting new rocky, temperate planets around red dwarfs using the SPECULOOS telescopes, and (2) observing these planets with JWST to assess the presence and composition of their atmospheres or determine the nature of their surfaces. In this talk, I will share insights into these areas of my work, including the discovery of the new SPECULOOS-3b exoplanetary system and the detailed characterization of TRAPPIST-1's innermost planets using JWST data. I will also discuss stellar contamination caused by photometric heterogeneities, which represents the primary challenge in characterizing planets around red dwarfs in transit, along with various strategies to overcome this obstacle. Finally, I will offer some perspectives on the future study of rocky planets orbiting red dwarfs. WARNING: the first minutes of the recording are missing. |
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Tuesday October 8 2024 | Lorenzo DUCCI (University of Tübingen)
Accretion anomalies: a journey through fast transients phenomena and accretion instabilities in X-ray binaries Summary [click here] Accretion onto compact objects, such as black holes and neutron stars, is a fundamental process in astrophysics, powering some of the most luminous objects in the universe. In X-ray binaries, the accretion of matter from a companion star onto the compact object leads to the emission of intense X-ray radiation. While the general framework of accretion is well understood, there exist peculiar and enigmatic fast flaring activities exhibited by different types of X-ray binaries, which challenge our current understanding of these systems.
In this seminar, I will delve into the diverse flaring behaviors observed in various X-ray binaries, highlighting their extreme properties. I will then discuss the possible mechanisms that have been proposed to explain these events. By exploring them, we can gain insights into the underlying physics of accretion and the behavior of matter in extreme environments. Finally, I will outline the future prospects for studying these enigmatic events, including the potential for new discoveries with upcoming X-ray missions. |
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Tuesday October 15 2024 | Climate seminar Sophie SCHBATH (INRAE)
Parcours du laboratoire MaIAGE pour réduire l'empreinte environnementale de ses activités Summary [click here] L'unité MaIAGE a commencé à réfléchir et adopter des éco-gestes dès 2017. En 2020, elle réalise son premier bilan de gaz à effet de serre (GES) sur les données de 2019 et se porte volontaire en 2021 pour participer à l'expérimentation nationale du GDR Labo 1point5. Cette expérimentation, conduite sur 22 labos pilotes, vise à étudier la mise en mouvement des laboratoires vers une réduction de leurs émissions de GES et à explorer différents dispositifs de réduction. C'est dans ce cadre que l'unité a voté en 2022 un scénario visant 40% de réduction de ses GES en 2030 ; ce scénario s'accompagne d'un certain nombre de mesures obligatoires et/ou incitatives. C'est l'ensemble de ce parcours qui sera présenté lors de la présentation avec des exemples d'actions réalisées et de premiers résultats. |
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Wednesday October 16 2024 | Daisuke NAGAI (Yale University, USA)
Cosmology in the Era of Multi-Wavelength Astronomical Surveys Summary [click here] We are entering the golden age of multi-wavelength astronomical surveys. In the 2020s, a plethora of surveys (such as Euclid, eROSITA, Rubin-LSST, Simons Observatory, and CMB-S4) are underway or planned to provide unprecedented insights into cosmology and galaxy formation. In this talk, I will provide a brief overview of significant scientific opportunities and the notable challenges in the era of big data, with highlights on recent advances in computational modeling and the integral roles played by artificial intelligence and machine learning. |
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Tuesday November 5 2024 | Special seminar Jean-Marc BONNET-BIDAUD (DAp)
Chine, la civilisation du ciel transitoire Summary [click here] En Chine, dès l'époque classique des Han (IIe siècle AEC), le Ciel est conçu comme le miroir de la Terre, lieux d'évènements fortuits (éclipses, comètes, taches solaires, novae et supernovae,..) qui sont traqués et catalogués par une armée de scientifiques au service de l'empereur. De nombreux documents astronomiques ont ainsi été produits sur plusieurs millénaires, conservés ou récemment redécouverts par l’archéologie. C’est le cas notamment de la plus ancienne carte d’étoiles connue au Monde conçue il y a plus de 1200 et préservée dans un monastère bouddhique de la route de la Soie.
Avec la présentation de ces documents, on retracera les étapes, les instruments et les concepts qui ont mené l'astronomie chinoise ancienne à de nombreuses découvertes du ciel transitoire, à des époques où l’Europe n’envisageait encore le Ciel que comme une voute céleste fixe, éternelle et immuable. Avec ce recul de l’histoire, la mission franco-chinoise SVOM, qui va détecter et cataloguer les plus puissants phénomènes célestes variables, apparait aujourd'hui comme une continuation logique dans laquelle la France rejoint symboliquement la Chine dans la préoccupation du ciel transitoire. |
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Tuesday November 12 2024 | Joel ONG (Institute for Astronomy, University of Hawaii at Manoa)
New Views into Stellar Rotation from Asteroseismology Summary [click here] Asteroseismology — the analysis and interpretation of stellar oscillations — remains the only means by which we may directly inspect the properties of stellar interiors. I will describe new theoretical developments regarding the interpretation of asteroseismic rotational signatures in a particular class of variable stars — red-giant solar-like oscillators. While waves in most variable stars propagate in a single mode cavity, I formulate a description of multicavity oscillations in these red giants by way of a physical analogy where they behave as "acoustic molecular orbitals". I describe two recent observational results which this formalism has helped us to interpret: 1. the detection of engulfment signatures in evolved red giants, and 2. direct constraints on the misalignment angle between the rotational axis of the core and of the envelope, in the central star of a spin-orbit-misaligned planetary system. |
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Tuesday November 19 2024 | Postdoc seminar Jose Ignacio "Nacho" ANEZ LOPEZ (LFEMI)
Signatures of magnetic braking in Class 0 protostars ? Exploring the gas kinematics in magnetized models of low-mass star formation Summary [click here] The role of magnetic braking in regulating gravitational collapse and circumstellar disk during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disk sizes and the launching of high-velocity collimated jets, we aim at more direct tests of the magnetic braking in observations. The study of polarized dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process.
In the present work, we have used both non-ideal MHD models and synthetic observations from the radiative transfer of protostellar formation to put constraints on the magnetically-regulated disk formation scenario. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetized model shows a higher angular momentum redistribution above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O(2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetized model similar to the characteristics found in observational work. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from radial motions. |
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Tuesday January 10 2023 | Stéphane AUNE, Jean-Charles CUILLANDRE et Pierre ASTIER (CEA/SEDI, CEA/DAp et LPNHE)
20 years of MegaCam scientific operations on the Canada-France-Hawaii Telescope Summary [click here] Over its history CFHT has helped build or hosted dozens of instruments, operating from ultraviolet to mid-infrared wavelengths, attuned to Maunakea’s renowned observing conditions. One instrument, though, eclipses them all – MegaCam. It is in fact hard to imagine CFHT’s success today without the decisions made two decades ago to build what was then the largest digital focal plane in the world. MegaCam took engineering techniques to closely mount CCDs into a single camera to a whole new level. With its 1 degree field of view, MegaCam was able to tile the sky with images, creating rich and precise datasets on enormous scales that could be mined by the entire scientific community for decades. To date MegaCam has led to more peer reviewed science publications than all other instruments used at CFHT combined. It has been used to tune the model of Dark Energy and map vast regions of Dark Matter, unveil faint wisps of stars flung from interacting galaxies, reveal hundreds of free floating planets in star forming regions, and is currently directly contributing to a Stage IV dark energy measurement by providing the key ground-based photometry for the ESA Euclid space mission to derive photometric redshifts of hundreds of millions of galaxies over the northern sky. All of this is part of a revolution in wide field high-resolution optical imaging that CFHT pioneered for many years. Three speakers will offer a complete overview from technical aspects to the most impactful scientific result over these past two decades:
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Tuesday January 17 2023 | Ludovic PETITDEMANGE (LRA/LERMA)
Dynamo action and angular momentum transport in simulated stellar radiative zones Summary [click here] The evolution of a star is influenced by its internal rotation dynamics through transport and mixing mechanisms, which are poorly understood. Magnetic fields can play a role in transporting angular momentum and chemical elements, but the origin of magnetism in radiative stellar layers is unclear. Using global numerical simulations, we identify a subcritical transition to turbulence due to the generation of a magnetic dynamo. Our results have many of the properties of the theoretically-proposed Tayler-Spruit dynamo mechanism, which strongly enhances transport of angular momentum in radiative zones. It generates deep toroidal fields that are screened by the stellar outer layers. This mechanism could produce strong magnetic fields inside radiative stars, without an observable field on their surface. Magnetic fields generated by dynamo action appear as a process to trigger turbulence in stellar interiors. Depending on the parameters or initial conditions, we report different dynamo branches that could explain stellar magnetism and the rotation profiles observed for stars having a thick radiative envelope. |
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Tuesday March 14 2023 | Special seminar Marie GUÉGUEN (Université de Rennes)
Cosmology: a tension within code comparisons Summary [click here] Code comparisons in cosmology are often performed with the underlying objective of identifying predictions upon which different codes converge that can be interpreted as robust, free of artifacts, predictions. Such an objective assumes that code comparisons can be constructed in such a way that the two notions of convergence and of robustness collapse. But, in order to achieve such a goal, code comparisons of structure formation have to meet an important epistemic challenge: that of constructing their codes ensemble on the basis of codes that are as independent as possible, but also comparable. In this talk, I show that enforcing the latter often amounts to multiplying common idealizations that hinder the achievement of the former, and thus leads to code comparisons that include many unscrutinized sources of artefacts possibly similarly distorting the predictions of the model. As a result, I argue that in context of high uncertainties where the domain of verification and validation of simulations has shrunk to code comparisons, code comparisons are better and actually very efficient as exploratory tools, both for getting insights into the physics implemented and for breaking the epistemic opacity of numerical simulations. |
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Tuesday April 4 2023 | Andrei IGOSHEV (University of Leeds)
Magneto-thermal evolution of neutron stars Summary [click here] We perform first three-dimensional simulations of the magneto-thermal evolution using a spectral MHD code for crust confined magnetic field configurations. Our results show that presence of strong toroidal magnetic field in magnetars is necessary to explain their quiescent thermal emission, in particular a formation of a single hot spot. Using our thermal maps we are able to explain light curves of 10 out of 19 magnetars in quiescence. In the case of the central compact objects, we test the configuration of magnetic field formed as a result of stochastic dynamo. Such a magnetic field consists of multiple randomly orientated loops of magnetic field. Surface thermal map is becoming patchy and includes multiple hot and cold regions which are always observed simultaneously. The global dipolar field slowly formed as a result of the Hall and Ohmic evolution. In our simulations we see 5-10% pulsed fraction and difference of two times in temperature between hot and cold regions typical for observations of the central compact objects. We also study off-centred dipole configurations and found that they decay over time. |
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Tuesday April 11 2023 | Anne VERHAMME (Université de Genève)
Searching for the sources of Cosmic Reionisation Summary [click here] Over the past 5 years, we have made tremendous progress on both direct detections of the escape of ionizing radiation from galaxies, over a broad range of redshifts (and instruments), and the tests and validations of indirect probes of the escape of ionizing radiation from galaxies, both from observations and simulations. I will review these recent achievements, and describe the next steps to understand the nature of the sources of reionisation. |
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Tuesday April 18 2023 | Judith IRWIN (Queen's University, Canada)
CHANG-ES — Past and Future Summary [click here] CHANG-ES (Continuum Halos in Nearby Galaxies — an EVLA Survey) is a project to observe 35 nearby galaxies that are edge-on to the line of sight to focus on their radio halos and the disk-halo connection. Wide-band VLA observations at L-band (1.5 GHz) and C-band (6.0 GHz) have provided opportunities to study in-band spectral indices, and observations in all four Stokes parameters with Rotation Measure Synthesis has led to a new understanding of the structure of kpc-scale magnetic fields in disk galaxies. This talk will highlight some of the results of the project and look to the future, as newly completed S-band (3.0 GHz) observations have filled in the L-band to C-band gap and led to the widest contiguous frequency coverage yet seen for galaxies. |
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Tuesday May 16 2023 | Special seminar Vianney LEBOUTEILLER (LFEMI)
Soft-Skilling in Fundamental Research Summary [click here] Artificial intelligence (AI) holds more and more importance in our lives and in our work. While AI undeniably provides a smart and useful companion, the current debates of whether it may replace us altogether in our tasks highlights the role and importance of social interactions and emotions in the work environment at large.
In this non-astrophysics seminar, I will present a recent study whose goal is to identify socio-professional (a.k.a. transversal, transferable, or generally "soft") skills that are critical in the transformation and innovation of companies/industries. As it turns out, most of these skills are actually acquired or at least strengthened throughout a career in fundamental research, including during the PhD. This is not surprising as transformation, innovation, but also breakthroughs, creativity etc... are common motivations in both worlds in which we seek answers to questions but we also seek questions themselves, within a complex environment and network of people. Many difficulties arise when dealing with soft skills, however: identifying them, acquiring and/or realizing they have been acquired, improving them, measuring them, making them valuable, and convincing other people they have been acquired. All in all, while most soft skills are well-known, the process of sorting, grouping, ranking them is necessary to set reference frameworks that can be acknowledged by most people. The purpose of this talk is to describe what soft skills are, how they are an integral part of research, and preliminary thoughts on how they can be applied/converted to non-academic world. |
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Tuesday June 6 2023 | Postdoc seminar Carlos GÓMEZ GUIJARRO (LCEG)
Understanding the role of compact star formation in galaxy evolution with ALMA and JWST Summary [click here] Massive elliptical galaxies in the local universe appear to have their high-redshift analogs in the form of extremely compact quiescent galaxies. Therefore, it seems that compact star formation appears to play a pivotal role in the evolutionary pathways of massive galaxies across cosmic history. However, it remains to be understood what this role is in the broader picture set by the main sequence and the scaling relations in galaxy evolution. From an ALMA survey at 1.1mm, we reveal that compact star formation appears to be the norm in massive star-forming galaxies, and sizes as extended as typical star-forming stellar disks are rare. A population of galaxies with modest star formation rates, but which exhibit extremely compact star formation with starburst-like depletion timescales unveils. Compact star formation appears as a physical driver of depletion timescales, gas fractions, and dust temperatures. Gas and star formation compression seems to be a mechanism that allows to hold their star formation rate even when their gas fractions are low and they are presumably on the way to quiescence. Another population of galaxies missed in the deep optical surveys but bright at far-IR/mm wavelengths unveils thanks to recent JWST observations. We present a study investigating the drivers of dust attenuation in massive galaxies in the JWST-era, showing how the stellar mass and morphology plays an important role, with evidence for more compact stellar profiles resulting in the obscuration of galaxies. |
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Tuesday June 13 2023 | Postdoc seminar Ugo LEBREUILLY (LMPA)
Gas and dust evolution during the formation of protoplanetary disks Summary [click here] Protoplanetary disks are the consequence of angular momentum conservation during the protostellar collapse. Their formation is a complex process which includes numerous physical effects (non-ideal MHD, stellar feedback, gas and dust interactions, turbulence…). In this seminar, I will
present our recent works to better understand the formation of these disks. In the first part of the talk, I will focus on their gas content. I will show how modelling simultaneously the large scales of star forming regions and the small scales of protoplanetary disks allows us to constrain the statistical properties (mass, radius, temperature…) of these disks. The second part of the talk will be dedicated to the study of dust evolution. In particular, I will stress the consequences of this process not only for the formation of disks, but also for the formation of planets. |
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Tuesday June 27 2023 | Julien AUBERT (IPGP)
Variations géomagnétiques rapides: un nouveau message émis par le noyau de la Terre Summary [click here] Le signal géomagnétique est une source riche d’informations sur la structure, la dynamique interne et l’histoire de notre planète. La production du champ magnétique Terrestre par effet dynamo dans le noyau externe implique une large disparité d’échelle spatiales et plus spécifiquement temporelles, s’étalant sur une gamme allant de l’année au milliard d’années. Depuis la mise en place d’observatoires magnétiques à la surface émergée de la Terre, l’attention s’est portée sur l’explication des variations à l’échelle du siècle, qui sont liées aux mouvements de convection dans le noyau. Depuis une vingtaine d’années, une couverture satellitaire globale et continue a cependant mis en évidence des variations de l’année à la dizaine d’années, dont l’origine est débattue. Ces nouvelles données ouvrent une fenêtre sur des phénomènes magnétohydrodynamiques rapides, de nature ondulatoire, en interaction avec la convection lente dans le noyau. Dans cet exposé, je présenterai les défis posés par la simulation conjointe des deux phénomènes ainsi que les avancées récentes et applications géophysiques potentielles. |
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Thursday June 29 2023 | Julien DE WIT (Department of Earth, Atmospheric and Planetary Science, MIT, USA)
Robustly Characterizing Exoplanets in a New Era of High-Precision Transit Measurements Summary [click here] With a new-generation of great observatories coming online this decade, unprecedented insights into exoplanets will soon be within reach. Observatories such as the James Webb Space Telescope (JWST) notably enable the study of atmospheres around terrestrial exoplanets and can reveal tri-dimensional structures in the atmospheres of their larger counterparts. Robustly leveraging new observations to reach such achievements will however require extra care as the models currently used may not be up to par with their precision.
During this presentation, I will introduce work done by MIT’s Disruptive Planets group and collaborators towards supporting the robust in-depth characterization of exoplanets. I will specifically discuss how not accounting for the true shape of a planet can lead to a misinterpretation of its interior properties as well as atmospheric structure; how the current state of our understanding of light-matter interactions can similarly affect our interpretation of planetary spectra and thus inferences regarding their atmospheric properties; and how the current state of emission spectrum models for stars may even prevent from disentangling between the contribution of a planet and its host star, to start with. I will also present possible ways to address these challenges. I will end with a step-by-step roadmap to the robust characterization of temperate terrestrial planets with JWST, which includes habitability assessment. |
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Tuesday September 5 2023 | Postdoc seminar Lucie BAUMONT (LCS)
Galaxy Cluster Cosmology with fgas Summary [click here] The current Standard Model of Cosmology has successfully explained many phenomena, but it predicts that the majority of the Universe consists of dark matter and dark energy, whose properties are poorly understood. Because huge volumes collapse to form galaxy clusters, the largest known gravitationally bound structures, they are an ideal laboratory to study the Dark Universe. In fact, the ratio of baryonic matter to total matter in a massive cluster, fgas, can be considered representative of the matter content of the Universe as a whole. Measurements of fgas from the heaviest, dynamically relaxed galaxy clusters place powerful constraints on cosmological parameters as well as the dark energy equation of state. I will discuss constraints derived from fgas measurements using a multi-wavelength set of X-ray and optical data and provide outlook on the future of this measurement in the age of precision cosmology. |
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Tuesday September 12 2023 | Alexander MOISEEV (NASA/GSFC and University of Maryland)
New Mission Concept: Compton Telescope with Coded Aperture Mask, and its Science Perspectives Summary [click here] The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel Explorer-class concept for a next-generation telescope covering the poorly explored hard X-ray and soft gamma-ray energies. This concept builds upon the heritage of past and current missions, improving sensitivity and, very importantly, angular resolution. GECCO uses the combined Coded Aperture Mask and Compton telescope techniques to employ the benefits of both: superior angular resolution provided by the deployable Coded Aperture Mask, and good background rejection and wide field-of-view (FoV) provided by the Compton telescope. It is being developed at NASA/GSFC in collaboration with other US and foreign institutions. GECCO observations will extend arcminute angular resolution to high-energy images of the Galactic plane, combining the
spectral capabilities of INTEGRAL/IBIS and the x-ray imaging of NuSTAR and eROSITA, and will make a bridge to the Fermi-LAT observations, enabling a broad potential for discoveries in the MeV γ-ray sky.
With the unprecedented angular resolution of the coded mask telescope combined with the sensitive, wide FoV Compton telescope, GECCO will focus on two main science objectives:
The GECCO design is based on the novel CZT Imaging calorimeter, which serves as a standalone Compton telescope and as a focal plane detector for the Coded Mask. It also is a powerful tool to measure the γ-radiation polarization. GECCO’s octagon-shaped active shield also serves as a powerful all-sky detector of gamma-ray bursts, prompting the instrument to slew towards the burst direction and localize it with the Coded Aperture’s arcminute accuracy. GECCO will operate in the 100 keV - 10 MeV energy range, with energy resolution of ~ 1% from 0.5 - 5 MeV. The Coded Aperture Mask provides the angular resolution of ∼1 arcmin with a 2 × 2 deg2 fully coded field-of-view, while the Compton telescope provides the angular resolution of 3◦ − 6◦ with a 60 × 60 deg2 field-of-view. The 3σ, 106 s sensitivity is expected to be about 10-5 MeV cm-2 s-1 over the entire energy range. Primary mode of observation is fixed pointing, with extended exposure of the regions of interest. However, as a standalone Compton telescope with wide FoV, Imaging Calorimeter will simultaneously provide wide-area sky exploration, significantly broadening GECCO’s observational scope. |
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Tuesday September 19 2023 | Arjen VAN DER WEL (Ghent University, Belgium)
Galaxy Evolution from z=3 to the Present Day: Recent Insights from JWST and VLT surveys Summary [click here] Thanks to an abundance of excellent observational data the evolution of the population of galaxies in terms of its stellar mass function, the distribution of star-formation across galaxies and their structural properties is known with good precision. The challenge we face, though, is to decipher the evolutionary paths of individual galaxies. Two roads to achieving this goal are 1) to reconstruct star-formation histories of individual galaxies through detailed modeling of high-quality spectra, and 2) attempt to define 'rules' for galaxy evolution that tell us how the collection of individual galaxies must evolve so that the population properties are reproduced. I will describe our recent progress along these lines, enabled by high-quality data from VLT spectroscopy of z~1 galaxies (the LEGA-C survey) and high-quality imaging data from HST (CANDELS) and JWST (CEERS). |
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Tuesday September 26 2023 | Postdoc seminar Adam FINLEY (LDE3)
Studying the whole Sun: from small-scale heating to large-scale dynamics Summary [click here] The Sun’s large-scale magnetic field undergoes periodic reversals due to dynamo-action in the solar interior, through which the Sun’s magnetic field regenerates. The emergence of new magnetic field at the solar surface, after buoyantly rising through the convection zone, is clearly
visible due to the formation of dark spots (sunspots). However, current models of the solar dynamo are unable to self-consistently capture the formation of sunspots, due to the range of pressure scale heights needed to include the photosphere. Thus, dynamo models remain disconnected from sunspot observations. The cyclic evolution of the Sun’s magnetic field also has a clear impact on the structure of the solar atmosphere and outflowing wind above. Similarly, linking the evolution of different scales, from the buffeting of convective motions in the photosphere to the dissipation of Alfven waves in the solar wind. Modern models of the Sun, therefore, require the combination of expertise from a range of interconnected subject areas. In this talk, I will highlight some of the recent work from the WholeSun ERC Synergy grant (https://wholesun.eu), which
brings together expertise from five different host institutions across Europe. These works range from assessing the observational signatures of toroidal flux generation, to modelling small-scale energy injection at the base of the solar wind, and finally, estimating the large-scale variation in coronal structure and rotation during the solar cycle. |
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Tuesday October 3 2023 | Postdoc seminar Lara PANTONI (LFEMI)
Dust millimetre emission in nearby galaxies with NIKA2 (IRAM-30m): major challenges and latest results of the IMEGIN Large Program Summary [click here] The millimetre part of the spectrum is one of the least explored parts of a galaxy’s spectral energy distribution (SED), yet it contains emissions from three fundamentally important physical processes. These processes are thermal emission from dust, free-free emission from ionized gas and synchrotron emission from relativistic charged particles moving in the galactic magnetic field. The NIKA2 camera (IRAM-30m telescope), observing at 1.15 mm and 2 mm, provides additional data points for input into the comprehensive SED models and allows us to:
During my presentation, I will focus on the major challenges linked with data processing, uncertainty propagation, and large-scale emission filtering in NIKA2 maps (due to atmosphere removal during the data reduction process). I will show and discuss the latest significant results on NGC891 (Katsioli et al. 2023); NGC4254 (Pantoni et al. in prep.); NGC2976 and NGC2146 (Ejlali et al. in prep.); millimetre morphology (Nersesian et al. in prep.); future perspectives/applications. |
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Wednesday October 4 2023 | Joint DAp-DPhP seminar Roland BACON (CRAL)
WST - The Wide Field Spectroscopic Telescope Summary [click here] The WST project aim to study and built an innovative 10-m class wide-field spectroscopic survey telescope (WST) in the southern hemisphere with simultaneous operation of a large field-of-view (5 sq. degree) and high multiplex (20,000) multi-object spectrograph facility with both medium and high resolution modes (MOS), and a giant panoramic integral field spectrograph (IFS). The ambitious WST top-level requirements place it far ahead of existing and planned facilities. In just its first 5 years of operation, the MOS will target 250 million galaxies and 25 million stars at medium resolution + 2 million stars at high resolution, and 4 billion spectra with the IFS. WST will achieve transformative results in most areas of astrophysics. The combination of MOS and IFS spectroscopic surveys is one of the key aspects of the project. It is very attractive because of the high complementarity between the two approaches. I will detail this innovative point using the example of the MOS and MUSE surveys performed in the CDFS region. The project aims to be the next major post-ELT project. It is supported by a large consortium of very experienced institutes plus ESO, representing 9 European countries and Australia. |
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Tuesday October 17 2023 | Postdoc seminar Sudip CHAKRABORTY (LEPCHE)
Unraveling the magnetic origin of dense non-equatorial ultrafast outflows in black hole X-ray binaries Summary [click here] Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Tuesday November 21 2023 | Recent hiree seminar Camila CORREA (LCEG)
Journey into the Unknown: Dark Matter, Observations of Galaxies, and the Path Forward Summary [click here] The nature of dark matter remains as one of the big unknowns of our time. The general expectation for the coming years is that Euclid, DESI, Rubin LSST, among others, will facilitate the production of wide-field galaxy surveys with exceptionally precise measurements, which will be crucial for unraveling the mysteries surrounding dark matter. We eagerly anticipate that the new observations will reveal deviations from the canonical cold collisionless dark matter paradigm, offering insights into its true nature. However, the key question lingers: will it really happen? What is missing now, that Euclid and others state-of-the-art facilities will change? In this seminar, I will strive to address these questions and review the latest status on dark matter searchers, in order to give you an impression of where we are in the search for dark matter and where we are going.
Throughout the seminar, I will also provide an overview of my work on the nature of dark matter and delve into the pivotal role that cosmological simulations of galaxy formation play in this quest. In the interpretation of data from both current and upcoming state-of-the-art observatories, cosmological simulations have emerged as indispensable tools. Simulations have convinced us of the success of LCDM over large scales, and have given us the capability of breaking the degeneracy driven by baryonic physics and dark matter models. However, can we really trust the outcomes of simulations? with their limited resolution and ad-hoc subgrid prescriptions for galaxy evolution. Maintaining a critical perspective on the observational data we work with and the simulated data we generate daily is crucial for advancing in this field. During the seminar, I will provide updates on the current challenges faced by cosmological simulations and highlight the progress they have achieved. I plan to be as efficient as possible to ensure time for a concluding session that hopefully leaves you with food for thought—an engaging discussion about the future. What steps lie ahead in the development of cosmological simulations? What about on the nature of dark matter? What synergies are needed to be forged between theoretical advancements and observational endeavours? And, importantly, what role will DAp play in this unfolding narrative? Let’s chat more on Tuesday 21 Nov. at 10 am. |
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Tuesday November 28 2023 | Elisa CHISARI (University of Oxford)
A rising tide: galaxy alignments in the Stage IV era Summary [click here] A new generation of galaxy surveys is at our doorstep. By mapping billions of galaxies over the sky, they will deliver precision constraints on the cosmological model. The significant gains in constraining power are accompanied by the need to develop more accurate models of galaxy shapes. Several physical effects will come into play that could be neglected or simplified in previous analyses. I will discuss the constraining power of upcoming surveys, focusing mostly on Rubin Observatory, and the modelling challenges they face. Amongst such challenges, I will focus mostly on the “intrinsic alignments” of galaxies: alignments between their shapes that are likely to arise from tidal interactions between them. I will describe some new modelling strategies that could help reduce their impact on cosmological constraints. Finally, I will present some opportunities that arise for using intrinsic alignments themselves as a probe of cosmology and galaxy evolution. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Wednesday December 6 2023 | Ute LISENFELD (University of Granada)
Molecular gas in galaxies in compact groups Summary [click here] Galaxies in compact groups live in an environment where constant interactions take place, both through tidal and through collisional processes. They are are therefore ideal objects to study the influence of the environment on galaxy properties and evolution, in particular the quenching of star formation.
The atomic hydrogen is extremely affected by this environment, with high deficiencies in many cases, whereas the molecular gas content is on average relatively normal in most galaxies. However, the properties of the molecular gas are drastically different if one selects those galaxies, based on their mid-infrared colours from the Spitzer and WISE satallites, that are in transitioning phase from active to quiescent which have have a considerably lower molecular-to-stellar mass and star formation efficiency. In my talk I will present result form observations of the molecular gas and star formation in galaxies in compact groups, both from single dish and interferometric observations, that show indications that in many cases the molecular is highly perturbed (and therefore unable to form stars) in compact groups. |
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Tuesday December 12 2023 | Stefano ANDREON (INAF-OA Brera, Milan, Italy)
First massive galaxy clusters emerging from the cosmic web at z~2 Summary [click here] In this talk I report upon our results on the intracluster medium (ICM) of two clusters at the time when first clusters start to emerge from the cosmic web, z~2. Results are derived from new, high resolution, deep SZ and X-ray data providing us with the measurement of the two most distant resolved pressure profiles. IDCSJ1426 cluster at z=1.75 has a core whose properties are not far from the final stage, while the remaining part of the cluster is experiencing a sizable gas, heat and entropy transfer. JKCS041 at z=1.80 is caught just after a major merger event as evidenced by its SZ-X-ray peak offset, its low central pressure, and its low Compton-Y parameter compared to its WL mass. Comparison with plausible descendents shows that its ICM will experience major changes at all radii. |
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Tuesday February 1 2022 | Pascal TREMBLIN (Maison de la Simulation)
Non-ideal self-gravity and cosmology: the importance of correlations in the dynamics of the large-scale structures of the Universe
Summary [click here] Inspired by the statistical mechanics of an ensemble of interacting particles (BBGKY hierarchy), we propose to account for small-scale inhomogeneities in self-gravitating astrophysical fluids by deriving a non-ideal Virial theorem and non-ideal Navier-Stokes equations using a decomposition of the gravitational force into a near- and far-field component. These equations involve the pair radial distribution function (similar to the two-point correlation function), similarly to the interaction energy and equation of state in liquids. Small-scale correlations lead to a non-ideal amplification of the gravitational interaction energy, whose omission leads to a missing mass problem, e.g., in galaxies and galaxy clusters. We also propose an extension of the Friedmann equations in the non-ideal regime. We estimate the non-ideal amplification factor of the gravitational interaction energy of the baryons to lie between 5 and 20, potentially explaining the observed value of the Hubble parameter. Within this framework, the acceleration of the expansion emerges naturally because of the increasing number of sub-structures induced by gravitational collapse, which increases their contribution to the total gravitational energy. A simple estimate predicts a non-ideal deceleration parameter qni~-1; this is potentially the first determination of the observed value based on an intuitively physical argument. We suggest that correlations and gravitational interactions could produce a transition to a viscous regime that can lead to flat rotation curves. This transition could also explain the dichotomy between (Keplerian) LSB elliptical galaxy and (non-Keplerian) spiral galaxy rotation profiles. Overall, our results demonstrate that non-ideal effects induced by inhomogeneities must be taken into account in order to properly determine the gravitational dynamics of galaxies and the larger scale universe. |
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Tuesday February 8 2022 | Special seminar Ingo WALDMANN (UCL)
Deep learning in exoplanet characterisation Summary [click here] The use of machine and deep learning is prevalent in many fields of science and industry and is now becoming more widespread in extrasolar planet and solar system sciences. Deep learning holds many potential advantages when it comes to modelling highly non-linear data, as well as speed improvements when compared to traditional analysis and modelling techniques. However, their often ‘black box’ nature and unintuitive decision processes, are a key hurdle to their broader adoption. In this seminar, I will give an overview of deep learning approaches used in exoplanet characterisation and discuss our recent work on developing Explainable AI (XAI) approaches. XAI is a rapidly developing field in machine learning and aims to make ‘black box’ models interpretable. By understanding how different neural net architectures learn to interpret atmospheric spectra, we can derive more robust prediction uncertainties as well as map information content as function of wavelength. As data and model complexities are bound to increase dramatically with the advent of JWST and ELT measurements, robust and interpretable deep learning models will become valuable tools in our data analysis repertoire. |
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Tuesday March 22 2022 | Benjamin WEHMEYER (CSFK, Budapest)
Galactic Chemical Evolution of rapid neutron capture process elements using special, rare classes of supernovae, and of short lived radioisotopes Summary [click here] The origin of the heaviest elements is still a matter of debate. For the rapid neutron capture process (r-process), multiple sites have been proposed, e.g., neutron star mergers and (sub-classes) of supernovae (e.g., magnetorotationally driven supernovae). R-process elements have been measured in a large fraction of metal-poor stars. Galactic archaeology studies show that the r-process abundances among these stars vary by over 2 orders of magnitude. On the other hand, abundances in stars with solar-like metallicity do not differ greatly. This leads to two major open questions:
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Tuesday April 5 2022 | Barbara OLMI (INAF, Italy)
Modeling Pulsar Wind Nebulae through their evolutionary phases Summary [click here] Pulsar wind nebulae are fascinating systems, powered by the central rotating compact star, emanating a wind in the form of a relativistic, magnetized, and cold plasma that fills the nebula. They are visible as bright non-thermal sources in a very broad range of energies, from radio to gamma-rays. Observed morphologies vary with the evolutionary phase, with middle-aged and old systems strongly affected by the interaction with the ambient medium. Modeling of these sources requires some carefulness when going through the various phases, with a comprehensive description still lacking.
Pulsar wind nebulae had been for a long time thought to contribute substantially to the positron excess in the CR spectrum at Earth -- potentially being the primary sources. In the last years, numerous evidence for efficient particle leakage by aged nebulae had been collected, showing up as quasi-monochromatic misaligned jets at X-rays in some cases, or in the form of extended TeV halos in others, reanimating somehow the interest in this class of objects. Here I will review our present knowledge of pulsar wind nebulae models through their different ages. |
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Tuesday April 19 2022 | Sacha BRUN et Olivier LIMOUSIN (DAp)
Solar Orbiter: the heliospheric explorer Summary [click here] Solar Orbiter - ESA M1 Mission - has entered its scientific phase at the end of November 2021, after a cruise phase of more than 18 months (and 2.2 Billion km) and the commissioning of the 10 instruments on board. CEA/IRFU played a key role in this mission, by providing the focal plane detector array of the X-ray telescope, STIX, based on in-house Caliste technology. In the meantime, solar physicists have been busy preparing the pipelines needed to handle the data sent by the instrument, and developing high performance numerical simulations of the Sun.
In this 2-voice seminar, we will relate the first 26 months of activities of the mission, covering both STIX calibration, its first light, the first solar flare detected, as well as the development of associated numerical simulations of the Sun and optimal scientific processing of instrumental data sent as the spacecraft gets closer and closer to the Sun (last perihelion was on 26 March 2022 at 0.32 AU), while in the meantime our star is increasing in intensity and in activity, with solar magnetic cycle 25 now well on its way. |
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Tuesday May 10 2022 | Group seminar Lev TITARCHUK (University of Ferrara, Italy)
Comptonization Problem and Its solution in Application to the Spectra of the Neutron Star and Black Hole Sources Summary [click here] In 2017 the work on the Comptonization (Sunyaev-Titarchuk) seen in the X-ray spectra of astrophysical sources was a candidate for the Nobel Prize in Physics. In this talk I provide all the details of the exciting prehistory of this topic and precise details of this discovery. The solution of this problem and its subsequent development and application to the spectra of accreting neutron star (NS) and black hole (BH) binaries reveals a lot of information on these objects. In particular, now we can unambiguously distinguish between a NS and a BH (Galactic or extragalactic) using correlations of their spectral indices vs mass accretion rate (or QPO frequency). I further demonstrate how we can determine a BH mass using this correlation. |
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Tuesday May 17 2022 | Recent hiree seminar Benjamin MAGNELLI (DAp)
A decade of the main-sequence of star-forming galaxies: New insights and perspectives on massive galaxy formation Summary [click here] The observed tight correlation between the star formation rate (SFR) and the stellar mass of star-forming galaxies (SFGs) is now well constrained over the last 10 Gyr of look-back time. This so-called main sequence (MS), whose normalization declines from z~3 to 0, is commonly interpreted as evidence that SFGs are evolving primarily through a steady and long star-forming mode, likely sustained by the cold gas accretion along the cosmic web. Over the last decade, a plethora of studies have investigated within this framework the physical properties of SFGs along and across the MS, establishing key scaling relations between, e.g., the stellar mass, gas content, and/or morphology of SFGs in the SFR-stellar mass plane. In this talk I will review past and recent observational evidences of this new MS paradigm and how it has shaped our understanding of the evolution of massive galaxies. Then, I will present the limitations of this simple paradigm, and in particular how it fails to explain the more diverse than anticipated population of MS galaxies (e.g., starburst hidden within the MS), the importance of secondary parameters (e.g., environment) and the transition of SFGs to quiescence. I will conclude by presenting future observational opportunities that can be used to investigate this hidden complexity within the main sequence and to further unveil the physics involved in the evolution of massive galaxies over cosmic time. |
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Tuesday June 14 2022 | Diego GÖTZ et Aline MEURIS (DAp)
The MXT telescope on board SVOM: a new tool for time-domain and multi-messenger astrophysics Summary [click here] The Sino-French SVOM (Space based astronomical Variable Object Monitor) mission is ready for launch in 2023. This mission is dedicated to the study of Gamma-Ray Bursts and other transient and variable sources of the high-energy sky. On board SVOM there will be four instruments, ECLAIRs and GRM, with large field of views, operating in the hard X- and gamma-ray domain, and two narrow field instruments: the Visible Telescope (VT) and the Microchannel X-ray Telescope (MXT). The MXT is a novel kind of compact and light instrument based of the « Lobster Eye » optical concept, coupled to a low noise state-of-the art X-ray camera, the latter being designed and manufactured at CEA Irfu. For a total mass of 42 kg and a total power of 60 W, this instrument is composed of an optics system, a telescope tube in carbon fiber, a radiator, a camera and a data processing unit. The 9 kg camera consists of a focal plane assembly with a detector assembly and thermoelectrical coolers, a front-end electronics assembly, a calibration wheel assembly and a support structure assembly. We will first review the SVOM scientific objectifs and how MXT will contribute to reach them. Then we will present the MXT design in more detail, focussing on the CEA contribution, and finally we will present the results of the calibration campaign performed in 2021 before the delivery of the telescope to Cnes. |
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Tuesday June 21 2022 | Recent hiree seminar Sandrine CODIS (DAp)
Modeling the birth and growth of the cosmic web Summary [click here] Starting from the largest scales, I will first describe how the cosmic web is woven across cosmic time into a gigantic bubble-like tapestry made of nodes, filaments, walls and voids. A particular emphasize will be put on the geometry and connectivity of this cosmic foam. Recent theoretical works aiming to precisely model the Universe on those mildly non-linear scales will be presented. In particular, I will identify a regime where large-deviation theory can be successfully implemented to predict the so-called count-in-cells statistics and describe promising cosmological applications for future galaxy surveys. The second part of the talk will focus on the birth and evolution of haloes and galaxies within these large cosmic highways. The highly anisotropic galactic environment set by the cosmic web will be shown to play a significant role in shaping them, an effect inducing large-scale galaxy alignments that are difficult to model but represent an important contamination for weak lensing experiments. |
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Tuesday September 6 2022 | Group seminar Nicolas SCEPI (CU Boulder)
Formation, Evolution and Radiation of Magnetized Accretion Disks around Black Holes Summary [click here] The impact of magnetic fields on the evolution and on the observational signatures of accretion disks is very uncertain. This uncertainty is mainly due to a lack of observational constraints on the magnetic field geometry or strength in accretion disks. However, even from a theoretical point of view our understanding of magnetized disks remains relatively poor. Indeed, analytic models of magnetized disks often need inputs from numerical simulations and numerical simulations of magnetized disks are difficult to perform and/or interpret. Because of this lack of magnetized disk models, standard disk models often reduce the magnetic field to a source of turbulence; turbulence through which the accretion can happen. While this simplification may hold for weakly magnetized disks, a large number of numerical simulations have shown that the role of a strong magnetic field goes far beyond producing turbulence. In particular, a strong magnetic field can produce powerful outflows, induce accretion through vertically elevated layers or non-axisymmetric structures, modify the time scales of accretion, enhance dissipation of gravitational energy in the disk and accelerate particles to very high energies. All of these effects dramatically affect the evolution and observational signature of accretion disks and open up new and exciting avenues to resolve outstanding problems of the standard accretion disk theory. In this talk, I will present an overview of my recent results on how strongly magnetized disks form, evolve and radiate. I will show in particular how strongly magnetized disks could explain events of very strong variability in AGNs, the flaring behavior of the Galactic center and the hardest emission in X-ray binaries. |
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Tuesday September 13 2022 | Special seminar Elisabeth KOHLER (CNRS)
Agir pour l’égalité professionnelle entre les femmes et les hommes Summary [click here] De la scolarité aux postes à responsabilité de nombreux stéréotypes persistent qui souvent entravent la carrière scientifique des femmes. L’objectif de ce séminaire est d’identifier ces biais et de présenter des exemples de mesures concrètes pour y remédier et mettre en œuvre des politiques d’égalité professionnelle entre les femmes et les hommes. Différents sujets seront abordés, tels que les critères d’évaluation, les actions de communication, le congé maternité et la parentalité, la lutte et la prévention contre les violences sexistes et sexuelles. |
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Tuesday October 11 2022 | Françoise COMBES (LERMA)
Overview of Cosmology with SKA, the Square Kilometer Array Summary [click here] SKA is a new technology radio-telescope array, about two orders of magnitude more sensitive and rapid in sky surveys than present instruments. It will be able to detect and measure the redshifts of billions of galaxies at the redshifts up to z=2, to probe through baryonic acoustic oscillations the nature of dark energy; it will probe the cosmic dawn of the universe, just afer recombination, and during the epoch of reionisation (z=6-15); it will be the unique instrument to map the atomic gas in high redshift galaxies, and determine the amount and distribution of dark matter in the early universe. With SKA-VLBI, it will unveil the accretion and feedback processes near super-massive black holes, and results from precursors will be shown. We will discuss these exciting perspectives, which will concretize at the end of the decade. |
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Thursday October 13 2022 | Group seminar Jordan PHILIDET (Max Planck Institute for Solar System Research, Göttingen)
Impact of turbulent convection on stellar oscillations Summary [click here] The study of the global oscillations of stars, which is at the heart of asteroseismology, has enabled us to make significant progress in our
understanding of stellar physics. In Sun-like stars, these oscillations are affected by the turbulent motions entailed by the convective instability in the envelope. In addition to modifying the resonant frequencies of the modes, turbulent convection is also responsible for their excitation, as well as part of their damping. As such, solar-like oscillations gives us access to crucial information about stellar convection, which constitutes one of the main obstacles towards better stellar modelling. However, in order to exploit the wealth of asteroseismic data at our disposal to better constrain the properties of stellar convection, it is necessary to theoretically model the relation between these properties and the asteroseismic observables (namely the amplitude of the modes, their lifetime, and the amount by which convection shifts their frequencies, referred to as surface effects).
In this talk, I will first focus on solar-like acoustic modes. Traditional approaches to study the effect of convection thereon are either based on parametric empirical formulations, or else on 3D simulations. These approaches show unavoidable limitations, among which the impossibility to realistically describe the full turbulent cascade, and especially the turbulent dissipation of kinetic energy. Here, I will present a new alternative theoretical framework designed to circumvent these limitations, based on Lagrangian stochastic models. I will demonstrate how, under reasonable assumptions, this sort of formalism can lead to simultaneous theoretical estimates for the amplitude, lifetime and surface effect of the acoustic modes, directly as a function of the turbulent fluctuations caused by convection, thus allowing to constrain turbulent convection models through the direct comparison of these estimates with observed mode properties. The second part of this talk will focus on another kind of oscillations, namely the inertial modes recently observed on the surface of the Sun. These modes propagate under the action of rotation, through the Coriolis acceleration, and, for the most part, are predicted to be stable, meaning that they are likely also excited by turbulent convection, just like acoustic modes. In order to test that hypothesis, I will present a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes, which are described by means of a 2D linear wave equation, under the β-plane approximation. Based on the general agreement between the predicted and observed inertial mode amplitudes, I will show that the (linearly stable) solar inertial modes are indeed excited by turbulent convection. This formalism also shows that the power in high azimuthal order spectra is not easily separable into individual modes, thus complicating the interpretation of the observations. |
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Tuesday November 8 2022 | Philippe Laurent, Aymeric Sauvageon, Jérôme Rodriguez (DAp)
INTEGRAL : 20 ans dans l'espace pour une aventure de plus de 35 ans Summary [click here] Le laboratoire INTErnational d'Astrophysique des Rayons Gamma (INTEGRAL) a été lancé le 17 octobre 2002 de Baikonour (Kasakstan). Depuis lors il est resté sur son orbite elliptique de haute excentricité (environ 3 jours) effectuant 2568 révolutions (au 8 novembre 2022) autour de la Terre pour environ 530 Ms (méga secondes) d'observations scientifiques. Cette mission de taille moyenne transporte deux instruments principaux opérant dans le domaine spectral des rayons X durs et des rayons gamma mous (20keV-10 MeV), dont l'optique est basée sur le concept de masques codés : l'un est dédié à l'analyse spectrale fine avec des capacités d'imagerie modérées (SPI), l'autre est dédié à l'imagerie fine (ou presque) avec des capacités spectrales modérées (IBIS).
Ces 20 ans dans l'espace sont le résultat d'au moins 15 ans de développement du projet avant qu'INTEGRAL puisse dévoiler le mystère du ciel à haute énergie. Le Dap a été profondément impliqué dans cette aventure de longue haleine, depuis le tout début du développement du concept de la mission (grande implication dans les deux instruments, ISGRI et SPI, participation au centre de données, développement de l'analyse s/w, suivi de la caméra) jusqu'à l'analyse actuelle des données en temps réel et des archives, avec un grand nombre d'articles publiés dans tous les domaines permis par les capacités instrumentales et même au-delà. Nous allons, dans ce séminaire, résumer ces plus de 35 ans en présentant divers aspects du projet/de la mission, en nous concentrant particulièrement sur ceux où l'implication du Dap a été cruciale. Nous commencerons par une vue d'ensemble de l'histoire de la mission, des principales caractéristiques instrumentales et des objectifs scientifiques au lancement, puis nous nous concentrerons sur la caméra ISGRI, la couche de détection 20-250 keV du télescope IBIS, qui est suivie au Dap tous les jours. ISGRI a été développée à l'IRFU et a obtenu la plupart des résultats d'INTEGRAL. Nous conclurons par un aperçu rapide et évidemment biaisé de quelques résultats scientifiques obtenus au cours de ces 20 années. Le séminaire sera présenté en Français avec des diapositives en Anglais par Philippe Laurent, Aymeric Sauvageon et Jérôme Rodriguez, bien humblement au nom d'un grand nombre de collègues anciens et actuels. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH VERSION INTEGRAL : 20 years in space and for a 35+ years adventure The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) has been launch on October 17th, 2002 from Baikonour (Kasakstan). Since then it has remained on its high-eccentricity elliptical orbit (about 3 days) performing 2568 revolutions (as of Nov. 8th, 2022) around the Earth for around 530 megaseconds of scientific observations. This medium sized mission caries two main instruments operating in the spectral domain of hard X-rays/Soft Gamma-rays (20keV-10 MeV), whose optics is based on the concept of coded masks: one is dedicated to fine spectral analysis with moderate imaging capabilities (SPI) the other is dedicated to fine(-ish) imaging with moderate spectral capabilities (IBIS). These 20 years in space are the results of at least 15 years of project development before INTEGRAL could unveil the mystery of the high-energy sky. The DAp has been deeply involved in this long-term adventure from the very beginning of the mission concept development (large involvement in both instruments, ISGRI and SPI, participation to the data centre, development of the s/w analysis, monitoring of the camera) to the current analysis of real time and archival data, with a large number of published papers in all fields allowed by the instrumental capabilities and even beyond. We will, in this seminary, summarised these 35 years+ by presenting various aspects of the project/mission, focusing especially on those where the involvement of the Dap has been crucial. This will start with an overall overview of the mission history, the main instrumental characteristics and scientific goals at launch, followed by a focus on the ISGRI camera, the 20-250 keV detector layer of the IBIS telescope, which is followed-up at DAp every days. ISGRI has been developed at IRFU and obtained most of the INTEGRAL results. We will conclude with a quick and obviously biased overview of some scientific results obtained over these 20 years. The talks will be given by Philippe Laurent, Aymeric Sauvageon, and Jérôme Rodriguez humbly on behalf of many, many former and current colleagues and presented in French with slides in English. |
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Tuesday November 15 2022 | Large project seminar Anaëlle MAURY (LFEMI)
Magnetic fields from star-forming cores to protostellar disks: a review of major contributions from the MagneticYSOs project Summary [click here] Wherever we have the means of observing them, magnetic fields are detected across the full spectrum of astrophysical environments, from our own Earth, to stars, and cosmological structures. Magnetic fields are also present at all scales and evolutionary stages of star-forming structures. They have long been suspected to play a key role in shaping the typical outcome of the star formation process, such as stellar mass, spin, and multiplicity, or even the fate of stars towards their ultimate stages.
In this talk, I will provide a global outlook on the progresses made in the recent years to characterize the role of magnetic fields during the embedded phases of the star formation process. Thanks to the development of observational capabilities and the parallel progress in numerical models capturing most of the important physics at work during star formation, the MagneticYSOs team successfully confronted detailed predictions of magnetized models to observational properties of the youngest protostars. I will present the physical processes and observational methods allowing to trace the magnetic field in embedded protostars, and review the main steps, success and limitations in comparing real observations to synthetic observations from the non-ideal MHD models. I will show how our work has shed light on the physical conditions required to ensure an efficient magnetic field coupling, and present unexpected results regarding the two main agents responsible for the coupling in star-forming cores: dust grains and ionized gas. Following this Ariane thread, I will argue our observational and theoretical findings support a novel scenario where the angular momentum problem for star formation may be actually “solved” not by the formation of large protoplanetary disks but by the combination of 1) lack of organized rotation motions at large envelope radii, 2) the inefficient angular momentum transport due to magnetic braking in the inner envelope (and angular momentum removed through rotating outflows generated by the presence of the magnetic field), and 3) a local origin of the angular momentum incorporated in the star–disk system. Reference review |
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Friday November 18 2022 | Group seminar Camille DIEZ (University of Tübingen)
Using Vela X-1 to understand accretion and wind structure in neutron star High-Mass X-ray Binaries (HMXBs) Summary [click here] The spectral and timing behaviour of HMXBs offers a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. The bright and persistent neutron star HMXB Vela X-1 is one of the key systems for such studies with both current and future instruments. It has a complex clumpy stellar wind, prominent cyclotron resonant scattering features (CRSFs) and strong flares. Understanding the variability of the systems on both short time scales of a few hundreds seconds and along its 9d orbit with current instruments enables us to make predictions for future observations with XRISM and Athena and to devise the best observational strategy for Vela X-1. Here, we analyse two new observations taken with NuSTAR and XMM-Newton at orbital phases ~0.5 and ~0.75 and follow the evolution of spectral parameters down to the pulse period (~300s) time-scale. The flux-dependency we observe in the spectral shape implies a change in the properties of the Comptonising plasma and the observed drop of the CRSF energy following a strong flare may indicate a change in the accretion geometry. The strong variability of absorption is due to the presence of a large-scale wind structure, such as accretion- and photoionisation wakes, combined with the variable line of sight as the neutron star moves along the orbit. In particular, we, for the first time, are able to trace the onset of the wakes with high time resolution and compare to predictions from simulations. |
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Tuesday November 22 2022 | Recent hiree seminar Antonio GARCIA MUÑOZ (DAp)
Exoplanet atmospheres: An anticipated revolution Summary [click here] The launch of the JWST less than a year ago is expected to set a turning point in exoplanet science, which is progressively transitioning from detection and population statistics to in-detail characterization of the exoplanets’ atmospheres. In this talk, I will present an admittedly biased
perspective of what this may represent for our understanding of exoplanets, and how ongoing theoretical work and future telescopes may build upon JWST’s legacy. |
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Friday November 25 2022 | Special seminar Leïla GODINAUD, Anne DECOURCHELLE, Fabio ACERO (DAp)
De Nova Stella : Séminaire d'anniversaire des 450 ans de la supernova de Tycho Summary [click here] En novembre 1572, l'observation de l'apparition d'une "nouvelle étoile" dans le ciel et les observations détaillées réalisées par Tycho Brahe ont transformé la façon dont nous voyons et comprenons notre Univers. Pour marquer l'événement, nous proposons un séminaire en trois parties allant de l'observation historique de l'événement jusqu'aux connaissances actuelles sur la supernova SN1572 et son vestige.
La conférence sera suivie d'un café gourmand dans le hall de la salle Galilée. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH De Nova Stella: 450th anniversary seminar for Tycho's supernova In November 1572, the observation of the apparition of a "new star" in the sky and the detailed observations made by Tycho Brahe transformed the way we see and understand our Universe. To mark the event we propose a three-part seminar ranging from the historical observation of the event to the current knowledge about the supernova SN1572 and its remnant. The conference will be followed by a gourmet coffee in the hall of the Galilee room. |
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Tuesday November 29 2022 | Elisa COSTANTINI (SRON, Netherlands)
The cold and dusty medium in our Galaxy as seen in the X-rays Summary [click here] X-rays have unique advantages in studying absorption and scattering from interstellar dust. For example, sharp and deep absorption features of Mg, Si, O and Fe, which are the building blocks of silicates, fall in the X-ray band. Present X-ray observatories already delivered to us interesting results, challenging the common paradigm on interstellar dust chemical and physical characteristics. Future instruments will open up an unexplored window, revealing the most dense environments of our Galaxy.
In this talk I will illustrate the state-of-art of our understanding of dust as seen in the X-rays, and future prospects, using for example, the upcoming XRISM satellite. |
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Tuesday December 6 2022 | David ALONSO (Oxford Department of Physics)
Modeling the birth and growth of the cosmic web Summary [click here] In the last 10-20 years we have been able to observe vast swathes of the Universe at different wavelengths, allowing us to build high-sensitivity maps of different projected cosmic properties. The statistical correlation between these properties and the density inhomogeneities that underlie the cosmic large-scale structures can then be used to reconstruct the spatial distribution of fundamental cosmological and astrophysical quantities, as well as their evolution in time. In this talk, I will describe a number of methods used to carry out this kind of tomographic reconstruction, present measurements of fundamental properties (structure growth, gas pressure, star formation rate density) resulting from their application to existing data, and discuss the potential of near-future "Stage-IV" experiments to improve on and benefit from these methods, in their quest to improve our understanding of fundamental physics. |
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Tuesday December 13 2022 | Recent hiree seminar Réza ANSARI (DAp)
Cosmology with 21cm Intensity Mapping Summary [click here] Intensity Mapping (IM) has been proposed about 15 years ago as an efficient technique to perform cosmological surveys. The 21cm hyperfine transition of neutral hydrogen can indeed be used to map the 3D distribution of matter in the universe, over a wide range of redshifts, from z=0 to z=3 or even z=6, bringing complementary information to the optical surveys.
Since then, few dedicated instruments have been built (CHIME, Tianlai, BINGO) to explore the feasibility of the method; Other more ambitious instruments, such as HIRAX, CHORD or BINGO will be commissioned in the coming years. Intensity mapping surveys are also envisaged for SKA, in addition to the classical HI source surveys. After presenting the principle of 21 intensity mapping, I will briefly discuss its cosmological promises, as well as some of the associated instrumental and scientific challenges. I will then present some of the results of ongoing observations, focusing on Tianlai, and on PAON4. Tianlai is an international project that operates two pathfinder instruments, a cylinder array and a parabolic array, built in Xinjiang, in western China. PAON4 is a small test interferometer, located in Nançay, used to explore some of the technical aspects of compact radio arrays, operating in transit mode. |
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Tuesday April 27 2021 | Frédéric BOURNAUD (Departement d’Astrophysique, IRFU, CEA-Saclay et Université Paris-Saclay)
Cosmological simulations of galaxy formation: toward a robust accounting of galaxy populations and star formation across the Universe? Summary [click here] Cosmological simulations of galaxy formation are reaching a high level of accuracy and can finely reproduce some of the main properties of galaxy populations: stellar masses, angular momentum, colors, etc. However, most galaxy formation simulations still fail to account for the detailed structure of galaxies and their global star formation history. Using high-resolution, idealized simulations of galactic dynamics and star formation, I will show that these disagreements are not cosmetic details but point toward a fundamental tension between observations and galaxy formation models. Historically, galaxy formation models predicted galaxies with unrealistically large stellar masses: in modern cosmological simulations, this issue is generally solved though energetic feedback from young stars and supermassive black holes. I will nevertheless show that feedback, as implemented in such simulations, is generally excessive, leading to the early and unrealistic exhaustion of interstellar gas reservoirs. Comparisons to idealized simulations and observations of galactic winds support the conclusion that energetic stellar and black hole feedback cannot be entirely responsible for the regulation of star formation and galaxy growth. Other physical processes likely emerge from sub-galactic scales in the interstellar medium, such as subtle coupling between galactic dynamics and star formation through instabilities and turbulence. Nevertheless, some cosmological simulations can now successfully describe the re-distribution of baryons from galaxies to the intergalactic medium, and I will show that these simulations are a crucial tool not just for galaxy formation but also for modern cosmological surveys. I will finally review how simulations of galaxy formation, evolution and star formation could gain strength in the years to come, in relation with the arrival of exascale supercomputers. |
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Tuesday November 23 2021 | Special seminar Michel TOULMONDE (SYRTE, Observatoire de Paris)
Émilie du Châtelet et les Principia de Newton Summary [click here] Selon Newton, la cause des mouvements des planètes et des comètes est la gravitation universelle qui crée les forces nécessaires pour que ces astres se déplacent sur des orbites elliptiques autour du Soleil. Il publie ce travail en 1687 dans sa grande œuvre les Philosophiæ naturalis principia mathematica, communément appelée les Principia.
Très admirative de ces idées nouvelles, Émilie Du Châtelet traduit en français le texte latin des Principia dans le but de les diffuser. Son action n'est pas solitaire : Maupertuis, Voltaire et Clairaut notamment vont la soutenir dans sa lourde tâche, car plusieurs savants français n’acceptent pas l’idée d’action à distance que serait la gravitation. Après le décès d'Émilie en 1749, Clairaut devient l'éditeur du manuscrit des Principes mathématiques de la Philosophie naturelle, mais l’ouvrage ne sera publié que dix ans plus tard, en 1759. À l’occasion de la parution d’une nouvelle édition critique de ce livre, on reverra les apports fondamentaux de Newton, puis on regardera le manuscrit d’Émilie Du Châtelet et on verra le rôle de Clairaut dans la parution tardive des Principes et du Commentaire écrits par « l’immortelle Émilie ». |
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Tuesday June 6 2023 | Postdoc seminar Carlos GÓMEZ GUIJARRO (LCEG)
Understanding the role of compact star formation in galaxy evolution with ALMA and JWST Summary [click here] Massive elliptical galaxies in the local universe appear to have their high-redshift analogs in the form of extremely compact quiescent galaxies. Therefore, it seems that compact star formation appears to play a pivotal role in the evolutionary pathways of massive galaxies across cosmic history. However, it remains to be understood what this role is in the broader picture set by the main sequence and the scaling relations in galaxy evolution. From an ALMA survey at 1.1mm, we reveal that compact star formation appears to be the norm in massive star-forming galaxies, and sizes as extended as typical star-forming stellar disks are rare. A population of galaxies with modest star formation rates, but which exhibit extremely compact star formation with starburst-like depletion timescales unveils. Compact star formation appears as a physical driver of depletion timescales, gas fractions, and dust temperatures. Gas and star formation compression seems to be a mechanism that allows to hold their star formation rate even when their gas fractions are low and they are presumably on the way to quiescence. Another population of galaxies missed in the deep optical surveys but bright at far-IR/mm wavelengths unveils thanks to recent JWST observations. We present a study investigating the drivers of dust attenuation in massive galaxies in the JWST-era, showing how the stellar mass and morphology plays an important role, with evidence for more compact stellar profiles resulting in the obscuration of galaxies. |
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Tuesday June 13 2023 | Postdoc seminar Ugo LEBREUILLY (LMPA)
Gas and dust evolution during the formation of protoplanetary disks Summary [click here] Protoplanetary disks are the consequence of angular momentum conservation during the protostellar collapse. Their formation is a complex process which includes numerous physical effects (non-ideal MHD, stellar feedback, gas and dust interactions, turbulence…). In this seminar, I will
present our recent works to better understand the formation of these disks. In the first part of the talk, I will focus on their gas content. I will show how modelling simultaneously the large scales of star forming regions and the small scales of protoplanetary disks allows us to constrain the statistical properties (mass, radius, temperature…) of these disks. The second part of the talk will be dedicated to the study of dust evolution. In particular, I will stress the consequences of this process not only for the formation of disks, but also for the formation of planets. |
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Tuesday September 5 2023 | Postdoc seminar Lucie BAUMONT (LCS)
Galaxy Cluster Cosmology with fgas Summary [click here] The current Standard Model of Cosmology has successfully explained many phenomena, but it predicts that the majority of the Universe consists of dark matter and dark energy, whose properties are poorly understood. Because huge volumes collapse to form galaxy clusters, the largest known gravitationally bound structures, they are an ideal laboratory to study the Dark Universe. In fact, the ratio of baryonic matter to total matter in a massive cluster, fgas, can be considered representative of the matter content of the Universe as a whole. Measurements of fgas from the heaviest, dynamically relaxed galaxy clusters place powerful constraints on cosmological parameters as well as the dark energy equation of state. I will discuss constraints derived from fgas measurements using a multi-wavelength set of X-ray and optical data and provide outlook on the future of this measurement in the age of precision cosmology. |
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Tuesday September 26 2023 | Postdoc seminar Adam FINLEY (LDE3)
Studying the whole Sun: from small-scale heating to large-scale dynamics Summary [click here] The Sun’s large-scale magnetic field undergoes periodic reversals due to dynamo-action in the solar interior, through which the Sun’s magnetic field regenerates. The emergence of new magnetic field at the solar surface, after buoyantly rising through the convection zone, is clearly
visible due to the formation of dark spots (sunspots). However, current models of the solar dynamo are unable to self-consistently capture the formation of sunspots, due to the range of pressure scale heights needed to include the photosphere. Thus, dynamo models remain disconnected from sunspot observations. The cyclic evolution of the Sun’s magnetic field also has a clear impact on the structure of the solar atmosphere and outflowing wind above. Similarly, linking the evolution of different scales, from the buffeting of convective motions in the photosphere to the dissipation of Alfven waves in the solar wind. Modern models of the Sun, therefore, require the combination of expertise from a range of interconnected subject areas. In this talk, I will highlight some of the recent work from the WholeSun ERC Synergy grant (https://wholesun.eu), which
brings together expertise from five different host institutions across Europe. These works range from assessing the observational signatures of toroidal flux generation, to modelling small-scale energy injection at the base of the solar wind, and finally, estimating the large-scale variation in coronal structure and rotation during the solar cycle. |
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Tuesday October 3 2023 | Postdoc seminar Lara PANTONI (LFEMI)
Dust millimetre emission in nearby galaxies with NIKA2 (IRAM-30m): major challenges and latest results of the IMEGIN Large Program Summary [click here] The millimetre part of the spectrum is one of the least explored parts of a galaxy’s spectral energy distribution (SED), yet it contains emissions from three fundamentally important physical processes. These processes are thermal emission from dust, free-free emission from ionized gas and synchrotron emission from relativistic charged particles moving in the galactic magnetic field. The NIKA2 camera (IRAM-30m telescope), observing at 1.15 mm and 2 mm, provides additional data points for input into the comprehensive SED models and allows us to:
During my presentation, I will focus on the major challenges linked with data processing, uncertainty propagation, and large-scale emission filtering in NIKA2 maps (due to atmosphere removal during the data reduction process). I will show and discuss the latest significant results on NGC891 (Katsioli et al. 2023); NGC4254 (Pantoni et al. in prep.); NGC2976 and NGC2146 (Ejlali et al. in prep.); millimetre morphology (Nersesian et al. in prep.); future perspectives/applications. |
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Tuesday October 17 2023 | Postdoc seminar Sudip CHAKRABORTY (LEPCHE)
Unraveling the magnetic origin of dense non-equatorial ultrafast outflows in black hole X-ray binaries Summary [click here] Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era. |
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Tuesday March 12 2024 | Postdoc seminar Arturo NUNEZ (LMPA)
The impact of baryonic physics in galaxy formation: Multi-scale approach to galaxy simulations, from the initial mass function to galaxies in a cosmological environment Summary [click here] The interplay of baryonic physics (star formation and feedback processes) in shaping galaxies and their host halos is a multiscale and multidisciplinary problem. Numerical simulations are key to understanding these processes as they cover phenomena that take place at a wide range of scales, some too small for most astrophysical observations but yet able to impact the shape and evolution of galaxies as a whole. However, no single simulation can address every aspect of this complex issue. A comprehensive approach is essential to integrate learnings from various types of simulations and compare them with observations. This includes understanding star formation histories in cosmological simulations, examining interstellar medium dynamics in comparison to simulations of individual galaxies, and exploring the non-universality in the stellar initial mass function in specific molecular cloud simulations. My goal is to discuss how simulations across different scales, together with detailed multiscale observations, collectively contribute to approaching the complexities of galaxy formation and evolution. All this while discussing current shortcomings and successes of high-resolution numerical simulations of galaxies and galactic environments.
WARNING: incomplete video recording. |
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Tuesday March 19 2024 | Postdoc seminar Miguel FERNANDES MOITA (LISIS)
The Large Italian X-ray facility (LARIX) and the advancements in Laue lens development Summary [click here] The LARIX (LARge Italian X-ray facility) is a multi-project facility situated in the Scientific-Technological Pole of the University of Ferrara, housed in an underground building featuring a 100-meter-long tunnel with two large experimental rooms on each side. Dedicated to the development and testing of X- and Gamma-ray astronomy instrumentation, LARIX hosts two beamlines: the 12-meter-long LARIX-A in experimental room A, suitable for linearity tests of hard X-ray detectors, reflectivity measurements of X-ray reflector samples, and ground calibrations; and the 50-meter-long LARIX-T installed in the tunnel, ideal for testing gamma-ray reflectors and low-weight gamma-ray detector prototypes when requiring low-divergence beams or lengthy beamlines. This presentation will provide an overview of both installations, their instrumentation, past projects, and opportunities for access through collaborations or transnational access programs like AHEAD. Furthermore, we will discuss recent advancements in Laue lens development that we did in LARIX, particularly the TRILL project, supported by ASI, aimed at advancing the technological readiness of Laue lenses. Future goals include the ASTENA mission concept, submitted to the ESA program 'Voyage 2050', featuring a narrow field telescope (NFT) based on a focusing Laue lens with an energy passband from 50 to 700 keV and a 20-meter focal length, promising breakthroughs in sensitivity and angular resolution in this energy band. Additionally, we will explore innovative concepts such as a high-energy Laue lens spectro-polarimeter, inspired by the recent success of the IXPE mission. |
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Tuesday April 30 2024 | Postdoc seminar Dinil PALAKKATHARAPPIL (LDE3)
Precise Stellar Age Determination of Stars with Gaia and TESS: Prospects and Insights for the upcoming PLATO and HAYDN Missions Summary [click here] Determining precise star ages is essential for insights into stellar, exoplanetary, and galactic evolution, but it remains a complex challenge. The two most precise techniques are (1) asteroseismology, which is the study of the internal structure of stars by means of intrinsic global oscillations, and (2) analysis of stellar clusters where stars are formed from the same molecular cloud at the same time, and the observed colour-magnitude
diagram is a strong diagnostic of the cluster age. In this talk, I will combine these two techniques to determine the age of intermediate age open cluster NGC 2477. We used astrometry and colour from Gaia to extract membership probabilities and colour information about the stars in clusters, while pulsating stars were found using TESS data. Observations are complemented with asteroseismic modelling using MESA and GYRE to obtain the precise age of the cluster. In the second part of the talk, I will detail my contribution to the development of the ground segment of the ESA M3 PLATO mission to obtain stellar mass, radius, and age as well as several rotation and magnetic activity indicators for which CEA is responsible. I will conclude by showcasing some applications of the PLATO pipeline using Kepler and TESS data. |
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Tuesday September 10 2024 | Postdoc seminar Maxime LOMBART (LFEMI)
How to treat dust coagulation/fragmentation in 3D hydrodynamic simulations ? Summary [click here] Particles coagulation and fragmentation are ubiquitous (raindrop formation, air pollution, combustion, polymerization, astrophysics) and mathematically described by the Smoluchowski coagulation and the fragmentation equations. Several processes such as gas/particles dynamics, chemical reaction and radiative transfer depends on the evolution of the particle size distribution governed by coagulation/fragmentation. Tracking the size evolution of particles in 3D simulation is key for understanding, for instance, cloud formation and planet formation. Therefore, these equations must be accurately solved while preserving computational costs, which is a tremendous numerical challenge. However, current algorithms for solving coagulation/fragmentation suffer from an over-diffusion in the conditions of 3D simulations. To tackle this challenge, we developed a discontinuous Galerkin scheme to efficiently solve the conservative form of the coagulation/fragmentation equation. In particular, we aim to perform the first 3D simulations of dusty protoplanetary discs and protostellar collapse that include realistic coagulation/fragmentation. |
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Tuesday September 17 2024 | Postdoc seminar Arghyadeep PAUL (LDE3)
Star-Planet Magnetic Interactions: From Solar System to Exoplanets Summary [click here] The interaction between stellar winds and planetary magnetospheres has been a focus of research for decades. Within the heliospheric context, star-planet interactions similar to that between the Sun and the Earth reshapes the planet’s local magnetic environment leading to the formation of magnetospheres. Magnetic reconnection within these magnetospheres gives rise to helical magnetic flux ropes known as flux transfer events (FTEs). We developed a two-way coupled magnetosphere-ionosphere model to investigate the impact of FTEs on the planet’s ionosphere. For a specific case of an Earth like planet, the field aligned currents generated by these FTEs closely resemble observations of discrete dayside auroral arcs, suggesting FTEs to be a probable cause. In the context of exoplanetary systems, star-planet interactions vary depending on the planet's orbital location. For close-in orbits located in the sub-Alfvénic stellar wind, the Poynting flux generated by star-planet interactions can propagate toward the star, giving rise to stellar chromospheric hotspots. Our current work characterises and quantifies the efficiency of this energy transfer
between the planet and the star, revealing that a significant portion of the energy never reaches the star and is reflected by the stellar transition region back toward the planet. The firm detection and characterisation of such magnetic interactions from observations of chromospheric hotspots would also lead to constraints on the amplitude of the magnetic field of exoplanets, to which we are blind so far. Future research, utilising the previously developed magnetosphere-ionosphere model, will also explore how the presence or absence of a planetary ionosphere influences the Poynting flux generated by the planet. |
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Tuesday November 19 2024 | Postdoc seminar Jose Ignacio "Nacho" ANEZ LOPEZ (LFEMI)
Signatures of magnetic braking in Class 0 protostars ? Exploring the gas kinematics in magnetized models of low-mass star formation Summary [click here] The role of magnetic braking in regulating gravitational collapse and circumstellar disk during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disk sizes and the launching of high-velocity collimated jets, we aim at more direct tests of the magnetic braking in observations. The study of polarized dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process.
In the present work, we have used both non-ideal MHD models and synthetic observations from the radiative transfer of protostellar formation to put constraints on the magnetically-regulated disk formation scenario. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetized model shows a higher angular momentum redistribution above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O(2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetized model similar to the characteristics found in observational work. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from radial motions. |
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Tuesday May 10 2022 | Group seminar Lev TITARCHUK (University of Ferrara, Italy)
Comptonization Problem and Its solution in Application to the Spectra of the Neutron Star and Black Hole Sources Summary [click here] In 2017 the work on the Comptonization (Sunyaev-Titarchuk) seen in the X-ray spectra of astrophysical sources was a candidate for the Nobel Prize in Physics. In this talk I provide all the details of the exciting prehistory of this topic and precise details of this discovery. The solution of this problem and its subsequent development and application to the spectra of accreting neutron star (NS) and black hole (BH) binaries reveals a lot of information on these objects. In particular, now we can unambiguously distinguish between a NS and a BH (Galactic or extragalactic) using correlations of their spectral indices vs mass accretion rate (or QPO frequency). I further demonstrate how we can determine a BH mass using this correlation. |
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Tuesday September 6 2022 | Group seminar Nicolas SCEPI (CU Boulder)
Formation, Evolution and Radiation of Magnetized Accretion Disks around Black Holes Summary [click here] The impact of magnetic fields on the evolution and on the observational signatures of accretion disks is very uncertain. This uncertainty is mainly due to a lack of observational constraints on the magnetic field geometry or strength in accretion disks. However, even from a theoretical point of view our understanding of magnetized disks remains relatively poor. Indeed, analytic models of magnetized disks often need inputs from numerical simulations and numerical simulations of magnetized disks are difficult to perform and/or interpret. Because of this lack of magnetized disk models, standard disk models often reduce the magnetic field to a source of turbulence; turbulence through which the accretion can happen. While this simplification may hold for weakly magnetized disks, a large number of numerical simulations have shown that the role of a strong magnetic field goes far beyond producing turbulence. In particular, a strong magnetic field can produce powerful outflows, induce accretion through vertically elevated layers or non-axisymmetric structures, modify the time scales of accretion, enhance dissipation of gravitational energy in the disk and accelerate particles to very high energies. All of these effects dramatically affect the evolution and observational signature of accretion disks and open up new and exciting avenues to resolve outstanding problems of the standard accretion disk theory. In this talk, I will present an overview of my recent results on how strongly magnetized disks form, evolve and radiate. I will show in particular how strongly magnetized disks could explain events of very strong variability in AGNs, the flaring behavior of the Galactic center and the hardest emission in X-ray binaries. |
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Thursday October 13 2022 | Group seminar Jordan PHILIDET (Max Planck Institute for Solar System Research, Göttingen)
Impact of turbulent convection on stellar oscillations Summary [click here] The study of the global oscillations of stars, which is at the heart of asteroseismology, has enabled us to make significant progress in our
understanding of stellar physics. In Sun-like stars, these oscillations are affected by the turbulent motions entailed by the convective instability in the envelope. In addition to modifying the resonant frequencies of the modes, turbulent convection is also responsible for their excitation, as well as part of their damping. As such, solar-like oscillations gives us access to crucial information about stellar convection, which constitutes one of the main obstacles towards better stellar modelling. However, in order to exploit the wealth of asteroseismic data at our disposal to better constrain the properties of stellar convection, it is necessary to theoretically model the relation between these properties and the asteroseismic observables (namely the amplitude of the modes, their lifetime, and the amount by which convection shifts their frequencies, referred to as surface effects).
In this talk, I will first focus on solar-like acoustic modes. Traditional approaches to study the effect of convection thereon are either based on parametric empirical formulations, or else on 3D simulations. These approaches show unavoidable limitations, among which the impossibility to realistically describe the full turbulent cascade, and especially the turbulent dissipation of kinetic energy. Here, I will present a new alternative theoretical framework designed to circumvent these limitations, based on Lagrangian stochastic models. I will demonstrate how, under reasonable assumptions, this sort of formalism can lead to simultaneous theoretical estimates for the amplitude, lifetime and surface effect of the acoustic modes, directly as a function of the turbulent fluctuations caused by convection, thus allowing to constrain turbulent convection models through the direct comparison of these estimates with observed mode properties. The second part of this talk will focus on another kind of oscillations, namely the inertial modes recently observed on the surface of the Sun. These modes propagate under the action of rotation, through the Coriolis acceleration, and, for the most part, are predicted to be stable, meaning that they are likely also excited by turbulent convection, just like acoustic modes. In order to test that hypothesis, I will present a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes, which are described by means of a 2D linear wave equation, under the β-plane approximation. Based on the general agreement between the predicted and observed inertial mode amplitudes, I will show that the (linearly stable) solar inertial modes are indeed excited by turbulent convection. This formalism also shows that the power in high azimuthal order spectra is not easily separable into individual modes, thus complicating the interpretation of the observations. |
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Friday November 18 2022 | Group seminar Camille DIEZ (University of Tübingen)
Using Vela X-1 to understand accretion and wind structure in neutron star High-Mass X-ray Binaries (HMXBs) Summary [click here] The spectral and timing behaviour of HMXBs offers a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. The bright and persistent neutron star HMXB Vela X-1 is one of the key systems for such studies with both current and future instruments. It has a complex clumpy stellar wind, prominent cyclotron resonant scattering features (CRSFs) and strong flares. Understanding the variability of the systems on both short time scales of a few hundreds seconds and along its 9d orbit with current instruments enables us to make predictions for future observations with XRISM and Athena and to devise the best observational strategy for Vela X-1. Here, we analyse two new observations taken with NuSTAR and XMM-Newton at orbital phases ~0.5 and ~0.75 and follow the evolution of spectral parameters down to the pulse period (~300s) time-scale. The flux-dependency we observe in the spectral shape implies a change in the properties of the Comptonising plasma and the observed drop of the CRSF energy following a strong flare may indicate a change in the accretion geometry. The strong variability of absorption is due to the presence of a large-scale wind structure, such as accretion- and photoionisation wakes, combined with the variable line of sight as the neutron star moves along the orbit. In particular, we, for the first time, are able to trace the onset of the wakes with high time resolution and compare to predictions from simulations. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Tuesday May 17 2022 | Recent hiree seminar Benjamin MAGNELLI (DAp)
A decade of the main-sequence of star-forming galaxies: New insights and perspectives on massive galaxy formation Summary [click here] The observed tight correlation between the star formation rate (SFR) and the stellar mass of star-forming galaxies (SFGs) is now well constrained over the last 10 Gyr of look-back time. This so-called main sequence (MS), whose normalization declines from z~3 to 0, is commonly interpreted as evidence that SFGs are evolving primarily through a steady and long star-forming mode, likely sustained by the cold gas accretion along the cosmic web. Over the last decade, a plethora of studies have investigated within this framework the physical properties of SFGs along and across the MS, establishing key scaling relations between, e.g., the stellar mass, gas content, and/or morphology of SFGs in the SFR-stellar mass plane. In this talk I will review past and recent observational evidences of this new MS paradigm and how it has shaped our understanding of the evolution of massive galaxies. Then, I will present the limitations of this simple paradigm, and in particular how it fails to explain the more diverse than anticipated population of MS galaxies (e.g., starburst hidden within the MS), the importance of secondary parameters (e.g., environment) and the transition of SFGs to quiescence. I will conclude by presenting future observational opportunities that can be used to investigate this hidden complexity within the main sequence and to further unveil the physics involved in the evolution of massive galaxies over cosmic time. |
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Tuesday June 21 2022 | Recent hiree seminar Sandrine CODIS (DAp)
Modeling the birth and growth of the cosmic web Summary [click here] Starting from the largest scales, I will first describe how the cosmic web is woven across cosmic time into a gigantic bubble-like tapestry made of nodes, filaments, walls and voids. A particular emphasize will be put on the geometry and connectivity of this cosmic foam. Recent theoretical works aiming to precisely model the Universe on those mildly non-linear scales will be presented. In particular, I will identify a regime where large-deviation theory can be successfully implemented to predict the so-called count-in-cells statistics and describe promising cosmological applications for future galaxy surveys. The second part of the talk will focus on the birth and evolution of haloes and galaxies within these large cosmic highways. The highly anisotropic galactic environment set by the cosmic web will be shown to play a significant role in shaping them, an effect inducing large-scale galaxy alignments that are difficult to model but represent an important contamination for weak lensing experiments. |
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Tuesday November 22 2022 | Recent hiree seminar Antonio GARCIA MUÑOZ (DAp)
Exoplanet atmospheres: An anticipated revolution Summary [click here] The launch of the JWST less than a year ago is expected to set a turning point in exoplanet science, which is progressively transitioning from detection and population statistics to in-detail characterization of the exoplanets’ atmospheres. In this talk, I will present an admittedly biased
perspective of what this may represent for our understanding of exoplanets, and how ongoing theoretical work and future telescopes may build upon JWST’s legacy. |
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Tuesday December 13 2022 | Recent hiree seminar Réza ANSARI (DAp)
Cosmology with 21cm Intensity Mapping Summary [click here] Intensity Mapping (IM) has been proposed about 15 years ago as an efficient technique to perform cosmological surveys. The 21cm hyperfine transition of neutral hydrogen can indeed be used to map the 3D distribution of matter in the universe, over a wide range of redshifts, from z=0 to z=3 or even z=6, bringing complementary information to the optical surveys.
Since then, few dedicated instruments have been built (CHIME, Tianlai, BINGO) to explore the feasibility of the method; Other more ambitious instruments, such as HIRAX, CHORD or BINGO will be commissioned in the coming years. Intensity mapping surveys are also envisaged for SKA, in addition to the classical HI source surveys. After presenting the principle of 21 intensity mapping, I will briefly discuss its cosmological promises, as well as some of the associated instrumental and scientific challenges. I will then present some of the results of ongoing observations, focusing on Tianlai, and on PAON4. Tianlai is an international project that operates two pathfinder instruments, a cylinder array and a parabolic array, built in Xinjiang, in western China. PAON4 is a small test interferometer, located in Nançay, used to explore some of the technical aspects of compact radio arrays, operating in transit mode. |
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Tuesday November 21 2023 | Recent hiree seminar Camila CORREA (LCEG)
Journey into the Unknown: Dark Matter, Observations of Galaxies, and the Path Forward Summary [click here] The nature of dark matter remains as one of the big unknowns of our time. The general expectation for the coming years is that Euclid, DESI, Rubin LSST, among others, will facilitate the production of wide-field galaxy surveys with exceptionally precise measurements, which will be crucial for unraveling the mysteries surrounding dark matter. We eagerly anticipate that the new observations will reveal deviations from the canonical cold collisionless dark matter paradigm, offering insights into its true nature. However, the key question lingers: will it really happen? What is missing now, that Euclid and others state-of-the-art facilities will change? In this seminar, I will strive to address these questions and review the latest status on dark matter searchers, in order to give you an impression of where we are in the search for dark matter and where we are going.
Throughout the seminar, I will also provide an overview of my work on the nature of dark matter and delve into the pivotal role that cosmological simulations of galaxy formation play in this quest. In the interpretation of data from both current and upcoming state-of-the-art observatories, cosmological simulations have emerged as indispensable tools. Simulations have convinced us of the success of LCDM over large scales, and have given us the capability of breaking the degeneracy driven by baryonic physics and dark matter models. However, can we really trust the outcomes of simulations? with their limited resolution and ad-hoc subgrid prescriptions for galaxy evolution. Maintaining a critical perspective on the observational data we work with and the simulated data we generate daily is crucial for advancing in this field. During the seminar, I will provide updates on the current challenges faced by cosmological simulations and highlight the progress they have achieved. I plan to be as efficient as possible to ensure time for a concluding session that hopefully leaves you with food for thought—an engaging discussion about the future. What steps lie ahead in the development of cosmological simulations? What about on the nature of dark matter? What synergies are needed to be forged between theoretical advancements and observational endeavours? And, importantly, what role will DAp play in this unfolding narrative? Let’s chat more on Tuesday 21 Nov. at 10 am. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Tuesday March 5 2024 | Recent hiree seminar Emmanuel BERTIN (LCEG)
Wide-field imaging meets deep learning: new challenges, new opportunities Summary [click here] Since the 19th century, wide-field imaging has significantly advanced numerous fields of Astrophysics, spanning from the study of solar system bodies to observational cosmology. Ongoing and future optical/near-infrared imaging surveys face many interesting data analysis challenges, especially in time domain astronomy, in a context where detector technology enable wide-field observations at increasingly high frame rates. Through concrete examples, I will show with how Deep Learning techniques offer promising solutions to address such challenges, and provide new scientific opportunities. |
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Tuesday May 14 2024 | Recent hiree seminar Damien TURPIN (DAp)
The high-energy and multi-messenger transient sky in the eyes of the SVOM mission Summary [click here] Over the past decades, the study of highly energetic transient phenomena has led to several scientific breakthroughs from the discovery of r-process nucleosynthesis production sites to the first credible identification of ultra-high-energy cosmic-ray accelerators. In a general manner, these explosive events allow us to study extreme physical regimes (extreme gravity, extreme magnetic fields, relativistic shocks) and are unique probes to study the Universe properties up to high redshift. In the last 10 years, an unprecedented amount of new generation instruments observing the sky at all wavelengths and with all astrophysical messengers have significantly contributed to enlarging our vision of the transient sky phenomena. This observational revolution initiated by the gravitational wave interferometers has forced the astrophysical community to rapidly adapt to the new challenges of multi-messenger astronomy. In June 2024, the French community will have a leading role in this young research domain with a strong implication for the French lab. in the upcoming SVOM mission. In this seminar, I will review some of the latest results obtained by the scientific community exploring the high-energy transient sky, especially with a focus on the sources of interest for the SVOM mission. I will show how important will be the SVOM mission contribution to this exciting field of research that has probably revealed only a subset of its richness. |
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Tuesday October 1 2024 | Recent hiree seminar Elsa DUCROT (LDE3)
Detection and characterization of rocky planets around ultra-cool stars Summary [click here] The launch of the James Webb Space Telescope (JWST) marked a significant milestone in the field of exoplanet research. For the first time, we can study the atmospheres of Earth-sized rocky planets—an opportunity that was previously unattainable. However, this is currently possible only around the coolest stars, known as red dwarfs. These stars are ideal targets for several reasons: (1) their smaller size makes it easier to detect and study small, transiting planets; (2) their lower luminosity leads to more frequent planetary transits for the same level of stellar irradiation; (3) they are the most abundant stars in the galaxy, and (4) planets orbiting them are more likely to be rocky planets with short orbital periods. In the near future, only such planetary systems are expected to produce signal-to-noise ratios (SNRs) high enough to confidently confirm or rule out the presence of atmospheres on potentially habitable planets.
In this context, my research focuses on: (1) detecting new rocky, temperate planets around red dwarfs using the SPECULOOS telescopes, and (2) observing these planets with JWST to assess the presence and composition of their atmospheres or determine the nature of their surfaces. In this talk, I will share insights into these areas of my work, including the discovery of the new SPECULOOS-3b exoplanetary system and the detailed characterization of TRAPPIST-1's innermost planets using JWST data. I will also discuss stellar contamination caused by photometric heterogeneities, which represents the primary challenge in characterizing planets around red dwarfs in transit, along with various strategies to overcome this obstacle. Finally, I will offer some perspectives on the future study of rocky planets orbiting red dwarfs. WARNING: the first minutes of the recording are missing. |
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Tuesday November 23 2021 | Special seminar Michel TOULMONDE (SYRTE, Observatoire de Paris)
Émilie du Châtelet et les Principia de Newton Summary [click here] Selon Newton, la cause des mouvements des planètes et des comètes est la gravitation universelle qui crée les forces nécessaires pour que ces astres se déplacent sur des orbites elliptiques autour du Soleil. Il publie ce travail en 1687 dans sa grande œuvre les Philosophiæ naturalis principia mathematica, communément appelée les Principia.
Très admirative de ces idées nouvelles, Émilie Du Châtelet traduit en français le texte latin des Principia dans le but de les diffuser. Son action n'est pas solitaire : Maupertuis, Voltaire et Clairaut notamment vont la soutenir dans sa lourde tâche, car plusieurs savants français n’acceptent pas l’idée d’action à distance que serait la gravitation. Après le décès d'Émilie en 1749, Clairaut devient l'éditeur du manuscrit des Principes mathématiques de la Philosophie naturelle, mais l’ouvrage ne sera publié que dix ans plus tard, en 1759. À l’occasion de la parution d’une nouvelle édition critique de ce livre, on reverra les apports fondamentaux de Newton, puis on regardera le manuscrit d’Émilie Du Châtelet et on verra le rôle de Clairaut dans la parution tardive des Principes et du Commentaire écrits par « l’immortelle Émilie ». |
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Tuesday February 8 2022 | Special seminar Ingo WALDMANN (UCL)
Deep learning in exoplanet characterisation Summary [click here] The use of machine and deep learning is prevalent in many fields of science and industry and is now becoming more widespread in extrasolar planet and solar system sciences. Deep learning holds many potential advantages when it comes to modelling highly non-linear data, as well as speed improvements when compared to traditional analysis and modelling techniques. However, their often ‘black box’ nature and unintuitive decision processes, are a key hurdle to their broader adoption. In this seminar, I will give an overview of deep learning approaches used in exoplanet characterisation and discuss our recent work on developing Explainable AI (XAI) approaches. XAI is a rapidly developing field in machine learning and aims to make ‘black box’ models interpretable. By understanding how different neural net architectures learn to interpret atmospheric spectra, we can derive more robust prediction uncertainties as well as map information content as function of wavelength. As data and model complexities are bound to increase dramatically with the advent of JWST and ELT measurements, robust and interpretable deep learning models will become valuable tools in our data analysis repertoire. |
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Tuesday September 13 2022 | Special seminar Elisabeth KOHLER (CNRS)
Agir pour l’égalité professionnelle entre les femmes et les hommes Summary [click here] De la scolarité aux postes à responsabilité de nombreux stéréotypes persistent qui souvent entravent la carrière scientifique des femmes. L’objectif de ce séminaire est d’identifier ces biais et de présenter des exemples de mesures concrètes pour y remédier et mettre en œuvre des politiques d’égalité professionnelle entre les femmes et les hommes. Différents sujets seront abordés, tels que les critères d’évaluation, les actions de communication, le congé maternité et la parentalité, la lutte et la prévention contre les violences sexistes et sexuelles. |
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Friday November 25 2022 | Special seminar Leïla GODINAUD, Anne DECOURCHELLE, Fabio ACERO (DAp)
De Nova Stella : Séminaire d'anniversaire des 450 ans de la supernova de Tycho Summary [click here] En novembre 1572, l'observation de l'apparition d'une "nouvelle étoile" dans le ciel et les observations détaillées réalisées par Tycho Brahe ont transformé la façon dont nous voyons et comprenons notre Univers. Pour marquer l'événement, nous proposons un séminaire en trois parties allant de l'observation historique de l'événement jusqu'aux connaissances actuelles sur la supernova SN1572 et son vestige.
La conférence sera suivie d'un café gourmand dans le hall de la salle Galilée. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH De Nova Stella: 450th anniversary seminar for Tycho's supernova In November 1572, the observation of the apparition of a "new star" in the sky and the detailed observations made by Tycho Brahe transformed the way we see and understand our Universe. To mark the event we propose a three-part seminar ranging from the historical observation of the event to the current knowledge about the supernova SN1572 and its remnant. The conference will be followed by a gourmet coffee in the hall of the Galilee room. |
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Tuesday March 14 2023 | Special seminar Marie GUÉGUEN (Université de Rennes)
Cosmology: a tension within code comparisons Summary [click here] Code comparisons in cosmology are often performed with the underlying objective of identifying predictions upon which different codes converge that can be interpreted as robust, free of artifacts, predictions. Such an objective assumes that code comparisons can be constructed in such a way that the two notions of convergence and of robustness collapse. But, in order to achieve such a goal, code comparisons of structure formation have to meet an important epistemic challenge: that of constructing their codes ensemble on the basis of codes that are as independent as possible, but also comparable. In this talk, I show that enforcing the latter often amounts to multiplying common idealizations that hinder the achievement of the former, and thus leads to code comparisons that include many unscrutinized sources of artefacts possibly similarly distorting the predictions of the model. As a result, I argue that in context of high uncertainties where the domain of verification and validation of simulations has shrunk to code comparisons, code comparisons are better and actually very efficient as exploratory tools, both for getting insights into the physics implemented and for breaking the epistemic opacity of numerical simulations. |
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Tuesday May 16 2023 | Special seminar Vianney LEBOUTEILLER (LFEMI)
Soft-Skilling in Fundamental Research Summary [click here] Artificial intelligence (AI) holds more and more importance in our lives and in our work. While AI undeniably provides a smart and useful companion, the current debates of whether it may replace us altogether in our tasks highlights the role and importance of social interactions and emotions in the work environment at large.
In this non-astrophysics seminar, I will present a recent study whose goal is to identify socio-professional (a.k.a. transversal, transferable, or generally "soft") skills that are critical in the transformation and innovation of companies/industries. As it turns out, most of these skills are actually acquired or at least strengthened throughout a career in fundamental research, including during the PhD. This is not surprising as transformation, innovation, but also breakthroughs, creativity etc... are common motivations in both worlds in which we seek answers to questions but we also seek questions themselves, within a complex environment and network of people. Many difficulties arise when dealing with soft skills, however: identifying them, acquiring and/or realizing they have been acquired, improving them, measuring them, making them valuable, and convincing other people they have been acquired. All in all, while most soft skills are well-known, the process of sorting, grouping, ranking them is necessary to set reference frameworks that can be acknowledged by most people. The purpose of this talk is to describe what soft skills are, how they are an integral part of research, and preliminary thoughts on how they can be applied/converted to non-academic world. |
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Tuesday November 5 2024 | Special seminar Jean-Marc BONNET-BIDAUD (DAp)
Chine, la civilisation du ciel transitoire Summary [click here] En Chine, dès l'époque classique des Han (IIe siècle AEC), le Ciel est conçu comme le miroir de la Terre, lieux d'évènements fortuits (éclipses, comètes, taches solaires, novae et supernovae,..) qui sont traqués et catalogués par une armée de scientifiques au service de l'empereur. De nombreux documents astronomiques ont ainsi été produits sur plusieurs millénaires, conservés ou récemment redécouverts par l’archéologie. C’est le cas notamment de la plus ancienne carte d’étoiles connue au Monde conçue il y a plus de 1200 et préservée dans un monastère bouddhique de la route de la Soie.
Avec la présentation de ces documents, on retracera les étapes, les instruments et les concepts qui ont mené l'astronomie chinoise ancienne à de nombreuses découvertes du ciel transitoire, à des époques où l’Europe n’envisageait encore le Ciel que comme une voute céleste fixe, éternelle et immuable. Avec ce recul de l’histoire, la mission franco-chinoise SVOM, qui va détecter et cataloguer les plus puissants phénomènes célestes variables, apparait aujourd'hui comme une continuation logique dans laquelle la France rejoint symboliquement la Chine dans la préoccupation du ciel transitoire. |
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Tuesday May 21 2024 | Climate seminar Céline GUIVARCH (École des Ponts ParisTech)
Le changement climatique vu à travers les lunettes d’une économiste Summary [click here] Les effets du changement climatique constituent une menace grandissante pour nos sociétés et nos économies; et agir face à cette menace nécessite de transformer nos façons de nous déplacer, nous loger, de produire et consommer. Autant de questions pour l'économie, pour analyser les implications économiques des trajectoires et politiques publiques d'atténuation du changement climatique. Ce séminaire proposera quelques incursions dans la recherche actuelle en économie du changement climatique, à partir d'un échantillon d'articles récents.
Le séminaire sera en français avec des slides en anglais. |
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Tuesday October 15 2024 | Climate seminar Sophie SCHBATH (INRAE)
Parcours du laboratoire MaIAGE pour réduire l'empreinte environnementale de ses activités Summary [click here] L'unité MaIAGE a commencé à réfléchir et adopter des éco-gestes dès 2017. En 2020, elle réalise son premier bilan de gaz à effet de serre (GES) sur les données de 2019 et se porte volontaire en 2021 pour participer à l'expérimentation nationale du GDR Labo 1point5. Cette expérimentation, conduite sur 22 labos pilotes, vise à étudier la mise en mouvement des laboratoires vers une réduction de leurs émissions de GES et à explorer différents dispositifs de réduction. C'est dans ce cadre que l'unité a voté en 2022 un scénario visant 40% de réduction de ses GES en 2030 ; ce scénario s'accompagne d'un certain nombre de mesures obligatoires et/ou incitatives. C'est l'ensemble de ce parcours qui sera présenté lors de la présentation avec des exemples d'actions réalisées et de premiers résultats. |
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Wednesday October 4 2023 | Joint DAp-DPhP seminar Roland BACON (CRAL)
WST - The Wide Field Spectroscopic Telescope Summary [click here] The WST project aim to study and built an innovative 10-m class wide-field spectroscopic survey telescope (WST) in the southern hemisphere with simultaneous operation of a large field-of-view (5 sq. degree) and high multiplex (20,000) multi-object spectrograph facility with both medium and high resolution modes (MOS), and a giant panoramic integral field spectrograph (IFS). The ambitious WST top-level requirements place it far ahead of existing and planned facilities. In just its first 5 years of operation, the MOS will target 250 million galaxies and 25 million stars at medium resolution + 2 million stars at high resolution, and 4 billion spectra with the IFS. WST will achieve transformative results in most areas of astrophysics. The combination of MOS and IFS spectroscopic surveys is one of the key aspects of the project. It is very attractive because of the high complementarity between the two approaches. I will detail this innovative point using the example of the MOS and MUSE surveys performed in the CDFS region. The project aims to be the next major post-ELT project. It is supported by a large consortium of very experienced institutes plus ESO, representing 9 European countries and Australia. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Tuesday May 21 2024 | Climate seminar Céline GUIVARCH (École des Ponts ParisTech)
Le changement climatique vu à travers les lunettes d’une économiste Summary [click here] Les effets du changement climatique constituent une menace grandissante pour nos sociétés et nos économies; et agir face à cette menace nécessite de transformer nos façons de nous déplacer, nous loger, de produire et consommer. Autant de questions pour l'économie, pour analyser les implications économiques des trajectoires et politiques publiques d'atténuation du changement climatique. Ce séminaire proposera quelques incursions dans la recherche actuelle en économie du changement climatique, à partir d'un échantillon d'articles récents.
Le séminaire sera en français avec des slides en anglais. |
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Tuesday June 11 2024 | Mustapha MEFTAH (LATMOS)
Les NanoSatellites au service de l’observation de la Terre et de l’univers Summary [click here] Le thème de la conférence portera sur l’observation de la Terre, le climat, les satellites et le NewSpace. Le 24 janvier 2021, une fusée Falcon 9 était lancée avec succès par SpaceX depuis Cap Canaveral, en Floride. Sous la coiffe du lanceur, il y avait 143 satellites. C’est à ce jour le plus grand nombre de satellites jamais déployés en une seule mission. Uvsq-Sat, premier nanosatellite français dédié à l’observation de variables climatiques essentielles, faisait partie de cette épopée. Depuis plus de trois ans, Uvsq-Sat observe la Terre et le Soleil de manière assidue. A 550 km d’altitude, ce satellite mesure la quantité d'énergie solaire réfléchie vers l'espace et la quantité d'énergie thermique émise par la Terre vers l’espace.
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Tuesday October 15 2024 | Climate seminar Sophie SCHBATH (INRAE)
Parcours du laboratoire MaIAGE pour réduire l'empreinte environnementale de ses activités Summary [click here] L'unité MaIAGE a commencé à réfléchir et adopter des éco-gestes dès 2017. En 2020, elle réalise son premier bilan de gaz à effet de serre (GES) sur les données de 2019 et se porte volontaire en 2021 pour participer à l'expérimentation nationale du GDR Labo 1point5. Cette expérimentation, conduite sur 22 labos pilotes, vise à étudier la mise en mouvement des laboratoires vers une réduction de leurs émissions de GES et à explorer différents dispositifs de réduction. C'est dans ce cadre que l'unité a voté en 2022 un scénario visant 40% de réduction de ses GES en 2030 ; ce scénario s'accompagne d'un certain nombre de mesures obligatoires et/ou incitatives. C'est l'ensemble de ce parcours qui sera présenté lors de la présentation avec des exemples d'actions réalisées et de premiers résultats. |
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Tuesday March 22 2022 | Benjamin WEHMEYER (CSFK, Budapest)
Galactic Chemical Evolution of rapid neutron capture process elements using special, rare classes of supernovae, and of short lived radioisotopes Summary [click here] The origin of the heaviest elements is still a matter of debate. For the rapid neutron capture process (r-process), multiple sites have been proposed, e.g., neutron star mergers and (sub-classes) of supernovae (e.g., magnetorotationally driven supernovae). R-process elements have been measured in a large fraction of metal-poor stars. Galactic archaeology studies show that the r-process abundances among these stars vary by over 2 orders of magnitude. On the other hand, abundances in stars with solar-like metallicity do not differ greatly. This leads to two major open questions:
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Tuesday April 5 2022 | Barbara OLMI (INAF, Italy)
Modeling Pulsar Wind Nebulae through their evolutionary phases Summary [click here] Pulsar wind nebulae are fascinating systems, powered by the central rotating compact star, emanating a wind in the form of a relativistic, magnetized, and cold plasma that fills the nebula. They are visible as bright non-thermal sources in a very broad range of energies, from radio to gamma-rays. Observed morphologies vary with the evolutionary phase, with middle-aged and old systems strongly affected by the interaction with the ambient medium. Modeling of these sources requires some carefulness when going through the various phases, with a comprehensive description still lacking.
Pulsar wind nebulae had been for a long time thought to contribute substantially to the positron excess in the CR spectrum at Earth -- potentially being the primary sources. In the last years, numerous evidence for efficient particle leakage by aged nebulae had been collected, showing up as quasi-monochromatic misaligned jets at X-rays in some cases, or in the form of extended TeV halos in others, reanimating somehow the interest in this class of objects. Here I will review our present knowledge of pulsar wind nebulae models through their different ages. |
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Tuesday May 10 2022 | Group seminar Lev TITARCHUK (University of Ferrara, Italy)
Comptonization Problem and Its solution in Application to the Spectra of the Neutron Star and Black Hole Sources Summary [click here] In 2017 the work on the Comptonization (Sunyaev-Titarchuk) seen in the X-ray spectra of astrophysical sources was a candidate for the Nobel Prize in Physics. In this talk I provide all the details of the exciting prehistory of this topic and precise details of this discovery. The solution of this problem and its subsequent development and application to the spectra of accreting neutron star (NS) and black hole (BH) binaries reveals a lot of information on these objects. In particular, now we can unambiguously distinguish between a NS and a BH (Galactic or extragalactic) using correlations of their spectral indices vs mass accretion rate (or QPO frequency). I further demonstrate how we can determine a BH mass using this correlation. |
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Tuesday September 6 2022 | Group seminar Nicolas SCEPI (CU Boulder)
Formation, Evolution and Radiation of Magnetized Accretion Disks around Black Holes Summary [click here] The impact of magnetic fields on the evolution and on the observational signatures of accretion disks is very uncertain. This uncertainty is mainly due to a lack of observational constraints on the magnetic field geometry or strength in accretion disks. However, even from a theoretical point of view our understanding of magnetized disks remains relatively poor. Indeed, analytic models of magnetized disks often need inputs from numerical simulations and numerical simulations of magnetized disks are difficult to perform and/or interpret. Because of this lack of magnetized disk models, standard disk models often reduce the magnetic field to a source of turbulence; turbulence through which the accretion can happen. While this simplification may hold for weakly magnetized disks, a large number of numerical simulations have shown that the role of a strong magnetic field goes far beyond producing turbulence. In particular, a strong magnetic field can produce powerful outflows, induce accretion through vertically elevated layers or non-axisymmetric structures, modify the time scales of accretion, enhance dissipation of gravitational energy in the disk and accelerate particles to very high energies. All of these effects dramatically affect the evolution and observational signature of accretion disks and open up new and exciting avenues to resolve outstanding problems of the standard accretion disk theory. In this talk, I will present an overview of my recent results on how strongly magnetized disks form, evolve and radiate. I will show in particular how strongly magnetized disks could explain events of very strong variability in AGNs, the flaring behavior of the Galactic center and the hardest emission in X-ray binaries. |
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Friday November 18 2022 | Group seminar Camille DIEZ (University of Tübingen)
Using Vela X-1 to understand accretion and wind structure in neutron star High-Mass X-ray Binaries (HMXBs) Summary [click here] The spectral and timing behaviour of HMXBs offers a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. The bright and persistent neutron star HMXB Vela X-1 is one of the key systems for such studies with both current and future instruments. It has a complex clumpy stellar wind, prominent cyclotron resonant scattering features (CRSFs) and strong flares. Understanding the variability of the systems on both short time scales of a few hundreds seconds and along its 9d orbit with current instruments enables us to make predictions for future observations with XRISM and Athena and to devise the best observational strategy for Vela X-1. Here, we analyse two new observations taken with NuSTAR and XMM-Newton at orbital phases ~0.5 and ~0.75 and follow the evolution of spectral parameters down to the pulse period (~300s) time-scale. The flux-dependency we observe in the spectral shape implies a change in the properties of the Comptonising plasma and the observed drop of the CRSF energy following a strong flare may indicate a change in the accretion geometry. The strong variability of absorption is due to the presence of a large-scale wind structure, such as accretion- and photoionisation wakes, combined with the variable line of sight as the neutron star moves along the orbit. In particular, we, for the first time, are able to trace the onset of the wakes with high time resolution and compare to predictions from simulations. |
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Tuesday April 4 2023 | Andrei IGOSHEV (University of Leeds)
Magneto-thermal evolution of neutron stars Summary [click here] We perform first three-dimensional simulations of the magneto-thermal evolution using a spectral MHD code for crust confined magnetic field configurations. Our results show that presence of strong toroidal magnetic field in magnetars is necessary to explain their quiescent thermal emission, in particular a formation of a single hot spot. Using our thermal maps we are able to explain light curves of 10 out of 19 magnetars in quiescence. In the case of the central compact objects, we test the configuration of magnetic field formed as a result of stochastic dynamo. Such a magnetic field consists of multiple randomly orientated loops of magnetic field. Surface thermal map is becoming patchy and includes multiple hot and cold regions which are always observed simultaneously. The global dipolar field slowly formed as a result of the Hall and Ohmic evolution. In our simulations we see 5-10% pulsed fraction and difference of two times in temperature between hot and cold regions typical for observations of the central compact objects. We also study off-centred dipole configurations and found that they decay over time. |
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Tuesday September 12 2023 | Alexander MOISEEV (NASA/GSFC and University of Maryland)
New Mission Concept: Compton Telescope with Coded Aperture Mask, and its Science Perspectives Summary [click here] The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel Explorer-class concept for a next-generation telescope covering the poorly explored hard X-ray and soft gamma-ray energies. This concept builds upon the heritage of past and current missions, improving sensitivity and, very importantly, angular resolution. GECCO uses the combined Coded Aperture Mask and Compton telescope techniques to employ the benefits of both: superior angular resolution provided by the deployable Coded Aperture Mask, and good background rejection and wide field-of-view (FoV) provided by the Compton telescope. It is being developed at NASA/GSFC in collaboration with other US and foreign institutions. GECCO observations will extend arcminute angular resolution to high-energy images of the Galactic plane, combining the
spectral capabilities of INTEGRAL/IBIS and the x-ray imaging of NuSTAR and eROSITA, and will make a bridge to the Fermi-LAT observations, enabling a broad potential for discoveries in the MeV γ-ray sky.
With the unprecedented angular resolution of the coded mask telescope combined with the sensitive, wide FoV Compton telescope, GECCO will focus on two main science objectives:
The GECCO design is based on the novel CZT Imaging calorimeter, which serves as a standalone Compton telescope and as a focal plane detector for the Coded Mask. It also is a powerful tool to measure the γ-radiation polarization. GECCO’s octagon-shaped active shield also serves as a powerful all-sky detector of gamma-ray bursts, prompting the instrument to slew towards the burst direction and localize it with the Coded Aperture’s arcminute accuracy. GECCO will operate in the 100 keV - 10 MeV energy range, with energy resolution of ~ 1% from 0.5 - 5 MeV. The Coded Aperture Mask provides the angular resolution of ∼1 arcmin with a 2 × 2 deg2 fully coded field-of-view, while the Compton telescope provides the angular resolution of 3◦ − 6◦ with a 60 × 60 deg2 field-of-view. The 3σ, 106 s sensitivity is expected to be about 10-5 MeV cm-2 s-1 over the entire energy range. Primary mode of observation is fixed pointing, with extended exposure of the regions of interest. However, as a standalone Compton telescope with wide FoV, Imaging Calorimeter will simultaneously provide wide-area sky exploration, significantly broadening GECCO’s observational scope. |
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Tuesday October 17 2023 | Postdoc seminar Sudip CHAKRABORTY (LEPCHE)
Unraveling the magnetic origin of dense non-equatorial ultrafast outflows in black hole X-ray binaries Summary [click here] Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era. |
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Tuesday May 14 2024 | Recent hiree seminar Damien TURPIN (DAp)
The high-energy and multi-messenger transient sky in the eyes of the SVOM mission Summary [click here] Over the past decades, the study of highly energetic transient phenomena has led to several scientific breakthroughs from the discovery of r-process nucleosynthesis production sites to the first credible identification of ultra-high-energy cosmic-ray accelerators. In a general manner, these explosive events allow us to study extreme physical regimes (extreme gravity, extreme magnetic fields, relativistic shocks) and are unique probes to study the Universe properties up to high redshift. In the last 10 years, an unprecedented amount of new generation instruments observing the sky at all wavelengths and with all astrophysical messengers have significantly contributed to enlarging our vision of the transient sky phenomena. This observational revolution initiated by the gravitational wave interferometers has forced the astrophysical community to rapidly adapt to the new challenges of multi-messenger astronomy. In June 2024, the French community will have a leading role in this young research domain with a strong implication for the French lab. in the upcoming SVOM mission. In this seminar, I will review some of the latest results obtained by the scientific community exploring the high-energy transient sky, especially with a focus on the sources of interest for the SVOM mission. I will show how important will be the SVOM mission contribution to this exciting field of research that has probably revealed only a subset of its richness. |
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Tuesday June 25 2024 | Jérôme RODRIGUEZ, Bertrand CORDIER & Damien TURPIN (DAp)
The SVOM Launch: En route for the transient hunt Summary [click here] The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a Sino-French mission planned to be launched on June 22nd 2024 from Xichang China. The launch of the satellite opens the time when scientists will step up and start exploiting the data. SVOM, however, is already a long-term project during which the involvement of engineers, developers, and scientists has already been huge. In this presentation I will overview some of the aspects of the mission, the scientific programs and aims of the project, but also recall its history and try and acknowledge the involvement of many colleagues which permitted the achievement of this fantastic machine. I will also introduce some of the novelties and specificities of the mission, in particular all the aspects dedicated to the time domain astronomy and real time analysis, implying the development of a dedicated science center, specific tools, and shifts for a certain of us as burst advocates. Finally we will (try to) have a zoom with those of us that have attended the launch in China, so that they can share with us their feeling of the launch and first impressions post launch, in this opening scientific exploitation era.
The talk will be given in French with slides in English. |
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Tuesday October 8 2024 | Lorenzo DUCCI (University of Tübingen)
Accretion anomalies: a journey through fast transients phenomena and accretion instabilities in X-ray binaries Summary [click here] Accretion onto compact objects, such as black holes and neutron stars, is a fundamental process in astrophysics, powering some of the most luminous objects in the universe. In X-ray binaries, the accretion of matter from a companion star onto the compact object leads to the emission of intense X-ray radiation. While the general framework of accretion is well understood, there exist peculiar and enigmatic fast flaring activities exhibited by different types of X-ray binaries, which challenge our current understanding of these systems.
In this seminar, I will delve into the diverse flaring behaviors observed in various X-ray binaries, highlighting their extreme properties. I will then discuss the possible mechanisms that have been proposed to explain these events. By exploring them, we can gain insights into the underlying physics of accretion and the behavior of matter in extreme environments. Finally, I will outline the future prospects for studying these enigmatic events, including the potential for new discoveries with upcoming X-ray missions. |
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Tuesday June 21 2022 | Recent hiree seminar Sandrine CODIS (DAp)
Modeling the birth and growth of the cosmic web Summary [click here] Starting from the largest scales, I will first describe how the cosmic web is woven across cosmic time into a gigantic bubble-like tapestry made of nodes, filaments, walls and voids. A particular emphasize will be put on the geometry and connectivity of this cosmic foam. Recent theoretical works aiming to precisely model the Universe on those mildly non-linear scales will be presented. In particular, I will identify a regime where large-deviation theory can be successfully implemented to predict the so-called count-in-cells statistics and describe promising cosmological applications for future galaxy surveys. The second part of the talk will focus on the birth and evolution of haloes and galaxies within these large cosmic highways. The highly anisotropic galactic environment set by the cosmic web will be shown to play a significant role in shaping them, an effect inducing large-scale galaxy alignments that are difficult to model but represent an important contamination for weak lensing experiments. |
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Tuesday October 11 2022 | Françoise COMBES (LERMA)
Overview of Cosmology with SKA, the Square Kilometer Array Summary [click here] SKA is a new technology radio-telescope array, about two orders of magnitude more sensitive and rapid in sky surveys than present instruments. It will be able to detect and measure the redshifts of billions of galaxies at the redshifts up to z=2, to probe through baryonic acoustic oscillations the nature of dark energy; it will probe the cosmic dawn of the universe, just afer recombination, and during the epoch of reionisation (z=6-15); it will be the unique instrument to map the atomic gas in high redshift galaxies, and determine the amount and distribution of dark matter in the early universe. With SKA-VLBI, it will unveil the accretion and feedback processes near super-massive black holes, and results from precursors will be shown. We will discuss these exciting perspectives, which will concretize at the end of the decade. |
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Tuesday December 6 2022 | David ALONSO (Oxford Department of Physics)
Modeling the birth and growth of the cosmic web Summary [click here] In the last 10-20 years we have been able to observe vast swathes of the Universe at different wavelengths, allowing us to build high-sensitivity maps of different projected cosmic properties. The statistical correlation between these properties and the density inhomogeneities that underlie the cosmic large-scale structures can then be used to reconstruct the spatial distribution of fundamental cosmological and astrophysical quantities, as well as their evolution in time. In this talk, I will describe a number of methods used to carry out this kind of tomographic reconstruction, present measurements of fundamental properties (structure growth, gas pressure, star formation rate density) resulting from their application to existing data, and discuss the potential of near-future "Stage-IV" experiments to improve on and benefit from these methods, in their quest to improve our understanding of fundamental physics. |
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Tuesday December 13 2022 | Recent hiree seminar Réza ANSARI (DAp)
Cosmology with 21cm Intensity Mapping Summary [click here] Intensity Mapping (IM) has been proposed about 15 years ago as an efficient technique to perform cosmological surveys. The 21cm hyperfine transition of neutral hydrogen can indeed be used to map the 3D distribution of matter in the universe, over a wide range of redshifts, from z=0 to z=3 or even z=6, bringing complementary information to the optical surveys.
Since then, few dedicated instruments have been built (CHIME, Tianlai, BINGO) to explore the feasibility of the method; Other more ambitious instruments, such as HIRAX, CHORD or BINGO will be commissioned in the coming years. Intensity mapping surveys are also envisaged for SKA, in addition to the classical HI source surveys. After presenting the principle of 21 intensity mapping, I will briefly discuss its cosmological promises, as well as some of the associated instrumental and scientific challenges. I will then present some of the results of ongoing observations, focusing on Tianlai, and on PAON4. Tianlai is an international project that operates two pathfinder instruments, a cylinder array and a parabolic array, built in Xinjiang, in western China. PAON4 is a small test interferometer, located in Nançay, used to explore some of the technical aspects of compact radio arrays, operating in transit mode. |
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Tuesday March 14 2023 | Special seminar Marie GUÉGUEN (Université de Rennes)
Cosmology: a tension within code comparisons Summary [click here] Code comparisons in cosmology are often performed with the underlying objective of identifying predictions upon which different codes converge that can be interpreted as robust, free of artifacts, predictions. Such an objective assumes that code comparisons can be constructed in such a way that the two notions of convergence and of robustness collapse. But, in order to achieve such a goal, code comparisons of structure formation have to meet an important epistemic challenge: that of constructing their codes ensemble on the basis of codes that are as independent as possible, but also comparable. In this talk, I show that enforcing the latter often amounts to multiplying common idealizations that hinder the achievement of the former, and thus leads to code comparisons that include many unscrutinized sources of artefacts possibly similarly distorting the predictions of the model. As a result, I argue that in context of high uncertainties where the domain of verification and validation of simulations has shrunk to code comparisons, code comparisons are better and actually very efficient as exploratory tools, both for getting insights into the physics implemented and for breaking the epistemic opacity of numerical simulations. |
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Tuesday April 11 2023 | Anne VERHAMME (Université de Genève)
Searching for the sources of Cosmic Reionisation Summary [click here] Over the past 5 years, we have made tremendous progress on both direct detections of the escape of ionizing radiation from galaxies, over a broad range of redshifts (and instruments), and the tests and validations of indirect probes of the escape of ionizing radiation from galaxies, both from observations and simulations. I will review these recent achievements, and describe the next steps to understand the nature of the sources of reionisation. |
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Tuesday September 5 2023 | Postdoc seminar Lucie BAUMONT (LCS)
Galaxy Cluster Cosmology with fgas Summary [click here] The current Standard Model of Cosmology has successfully explained many phenomena, but it predicts that the majority of the Universe consists of dark matter and dark energy, whose properties are poorly understood. Because huge volumes collapse to form galaxy clusters, the largest known gravitationally bound structures, they are an ideal laboratory to study the Dark Universe. In fact, the ratio of baryonic matter to total matter in a massive cluster, fgas, can be considered representative of the matter content of the Universe as a whole. Measurements of fgas from the heaviest, dynamically relaxed galaxy clusters place powerful constraints on cosmological parameters as well as the dark energy equation of state. I will discuss constraints derived from fgas measurements using a multi-wavelength set of X-ray and optical data and provide outlook on the future of this measurement in the age of precision cosmology. |
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Tuesday November 21 2023 | Recent hiree seminar Camila CORREA (LCEG)
Journey into the Unknown: Dark Matter, Observations of Galaxies, and the Path Forward Summary [click here] The nature of dark matter remains as one of the big unknowns of our time. The general expectation for the coming years is that Euclid, DESI, Rubin LSST, among others, will facilitate the production of wide-field galaxy surveys with exceptionally precise measurements, which will be crucial for unraveling the mysteries surrounding dark matter. We eagerly anticipate that the new observations will reveal deviations from the canonical cold collisionless dark matter paradigm, offering insights into its true nature. However, the key question lingers: will it really happen? What is missing now, that Euclid and others state-of-the-art facilities will change? In this seminar, I will strive to address these questions and review the latest status on dark matter searchers, in order to give you an impression of where we are in the search for dark matter and where we are going.
Throughout the seminar, I will also provide an overview of my work on the nature of dark matter and delve into the pivotal role that cosmological simulations of galaxy formation play in this quest. In the interpretation of data from both current and upcoming state-of-the-art observatories, cosmological simulations have emerged as indispensable tools. Simulations have convinced us of the success of LCDM over large scales, and have given us the capability of breaking the degeneracy driven by baryonic physics and dark matter models. However, can we really trust the outcomes of simulations? with their limited resolution and ad-hoc subgrid prescriptions for galaxy evolution. Maintaining a critical perspective on the observational data we work with and the simulated data we generate daily is crucial for advancing in this field. During the seminar, I will provide updates on the current challenges faced by cosmological simulations and highlight the progress they have achieved. I plan to be as efficient as possible to ensure time for a concluding session that hopefully leaves you with food for thought—an engaging discussion about the future. What steps lie ahead in the development of cosmological simulations? What about on the nature of dark matter? What synergies are needed to be forged between theoretical advancements and observational endeavours? And, importantly, what role will DAp play in this unfolding narrative? Let’s chat more on Tuesday 21 Nov. at 10 am. |
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Tuesday November 28 2023 | Elisa CHISARI (University of Oxford)
A rising tide: galaxy alignments in the Stage IV era Summary [click here] A new generation of galaxy surveys is at our doorstep. By mapping billions of galaxies over the sky, they will deliver precision constraints on the cosmological model. The significant gains in constraining power are accompanied by the need to develop more accurate models of galaxy shapes. Several physical effects will come into play that could be neglected or simplified in previous analyses. I will discuss the constraining power of upcoming surveys, focusing mostly on Rubin Observatory, and the modelling challenges they face. Amongst such challenges, I will focus mostly on the “intrinsic alignments” of galaxies: alignments between their shapes that are likely to arise from tidal interactions between them. I will describe some new modelling strategies that could help reduce their impact on cosmological constraints. Finally, I will present some opportunities that arise for using intrinsic alignments themselves as a probe of cosmology and galaxy evolution. |
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Tuesday December 12 2023 | Stefano ANDREON (INAF-OA Brera, Milan, Italy)
First massive galaxy clusters emerging from the cosmic web at z~2 Summary [click here] In this talk I report upon our results on the intracluster medium (ICM) of two clusters at the time when first clusters start to emerge from the cosmic web, z~2. Results are derived from new, high resolution, deep SZ and X-ray data providing us with the measurement of the two most distant resolved pressure profiles. IDCSJ1426 cluster at z=1.75 has a core whose properties are not far from the final stage, while the remaining part of the cluster is experiencing a sizable gas, heat and entropy transfer. JKCS041 at z=1.80 is caught just after a major merger event as evidenced by its SZ-X-ray peak offset, its low central pressure, and its low Compton-Y parameter compared to its WL mass. Comparison with plausible descendents shows that its ICM will experience major changes at all radii. |
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Tuesday January 9 2024 | Roya MOHAYAEE (IAP)
The Anomalous Dipole Summary [click here] Standard model of cosmology is based on the cosmological principle, which states that the Universe is statistically homogeneous and isotropic on large scales. Is this hypothesis supported by the observations ? After a historical survey of the field, I shall use the high redshift data from
radio galaxies and quasars to show that the early Universe does not seem to be isotropic and the rest frame of cosmic microwave background radiation does not coincide with the rest frame of distant sources. I shall also demonstrate that the cosmological principle is violated at a statistical significance of over 5-sigma. |
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Tuesday January 30 2024 | Florian PACAUD (University of Bonn)
Galaxy clusters and filaments in the first eROSITA all-Sky survey Summary [click here] Launched in July 2019, eROSITA was set to survey the sky in the soft X-ray band for four years and provide a much awaited update to the old ROSAT all-sky survey. In a few weeks, the collaboration will release the first all-sky map obtained from the first six month of observation. In this talk, I will present the instrument and its technical abilities. I will then explore the all-sky map and show selected results on individual local clusters and filaments. Finally, I will give a preview of the soon-to-be-released data products, including AGN and galaxy catalogs, and present some of the methods used for their cosmological analysis. |
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Tuesday February 27 2024 | Hakim ATEK (IAP)
Unveiling the faintest and the brightest galaxies at early times with JWST Summary [click here] The JWST is revolutionizing our understanding of the early Universe by unveiling a wealth of bright galaxies at z>9 and faint AGNs at z>5. I will present the latest constraints on the overabundance of UV-bright galaxies at z>9, which is 10-100 times higher than galaxy formation models. I will discuss to what extent recent theoretical efforts can reproduce such observations, and how future wide-area surveys such as Euclid will help put stronger constraints ion the bright-end of UVLF at z>8. On the other hand, faint galaxies, representing the building blocks of present-day galaxies, have eluded spectroscopic constraints, even with the deepest JWST campaigns so far. I will present the results of our UNCOVER survey, which combines ultra-deep NIRSpec spectroscopy with the strong lensing magnification of A2744 cluster. We characterize ultra-faint galaxies with intrinsic absolute magnitude between Muv=-17 and Muv=-15 at 6<z<8, and stellar masses down to 10^6 solar masses. I will discuss our plans to obtain the deepest observations on sky with the GLIMPSE program to the faintest galaxy population out to z=15 and beyond. |
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Tuesday March 12 2024 | Postdoc seminar Arturo NUNEZ (LMPA)
The impact of baryonic physics in galaxy formation: Multi-scale approach to galaxy simulations, from the initial mass function to galaxies in a cosmological environment Summary [click here] The interplay of baryonic physics (star formation and feedback processes) in shaping galaxies and their host halos is a multiscale and multidisciplinary problem. Numerical simulations are key to understanding these processes as they cover phenomena that take place at a wide range of scales, some too small for most astrophysical observations but yet able to impact the shape and evolution of galaxies as a whole. However, no single simulation can address every aspect of this complex issue. A comprehensive approach is essential to integrate learnings from various types of simulations and compare them with observations. This includes understanding star formation histories in cosmological simulations, examining interstellar medium dynamics in comparison to simulations of individual galaxies, and exploring the non-universality in the stellar initial mass function in specific molecular cloud simulations. My goal is to discuss how simulations across different scales, together with detailed multiscale observations, collectively contribute to approaching the complexities of galaxy formation and evolution. All this while discussing current shortcomings and successes of high-resolution numerical simulations of galaxies and galactic environments.
WARNING: incomplete video recording. |
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Tuesday April 2 2024 | James DUNLOP (University of Edinburgh)
Charting the emergence of the first galaxies with JWST Summary [click here] The James Webb Space Telescope (JWST) is transforming our view of galaxy formation and evolution in the young Universe. I will provide an overview of the latest results from the PRIMER survey, the largest JWST Cycle-1 “Galaxies” programme which, in combination with other public JWST imaging, is now enabling us to chart the early growth of the galaxy population back to within ~300 million years of the Big Bang. Specifically, I will present and discuss the first robust determination of the evolving galaxy luminosity function extending out to redshifts z~13, as well as new measurements of the galaxy stellar mass function reaching out to z~9. I will then attempt to interpret these results in the context of our current understanding of the evolving dark matter halo mass function and the efficiency with which galaxies are able to convert their baryons into stars. Finally, I will discuss the prospects for future progress with JWST and with the Atacama Large Millimeter Array (ALMA), including the possibility of uncovering the emergence of the first galaxies. |
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Tuesday April 23 2024 | Florent MERTENS (Observatoire de Paris)
Unveiling the Physics of the Cosmic Dawn and the Epoch of Reionisation in the SKA era Summary [click here] Direct observation of the Cosmic Dawn and Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are underway aiming at statistically detecting the 21-cm signal fluctuations from the EoR and CD, and paving the way for the SKA EoR CD experiment which will be capable of directly image the large-scale neutral hydrogen structures from these distance epochs. In this talk, I will present recent results from the LOFAR-EoR and NenuFAR Cosmic Dawn collaborations. After publishing the deepest upper limit on the signal power-spectra at z~9, which has made it possible to set constraints on the physics of the IGM during the EoR, the LOFAR-EoR team is progressing towards a deeper upper limit on a broader range of redshift. On the Cosmic Dawn front, the NenuFAR CD team recenty published a first upper limit on the signal power-spectra at z~20. These new results will be discussed, as well as the improvements in instrument calibration and foreground mitigation that enabled these achievements. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Wednesday October 16 2024 | Daisuke NAGAI (Yale University, USA)
Cosmology in the Era of Multi-Wavelength Astronomical Surveys Summary [click here] We are entering the golden age of multi-wavelength astronomical surveys. In the 2020s, a plethora of surveys (such as Euclid, eROSITA, Rubin-LSST, Simons Observatory, and CMB-S4) are underway or planned to provide unprecedented insights into cosmology and galaxy formation. In this talk, I will provide a brief overview of significant scientific opportunities and the notable challenges in the era of big data, with highlights on recent advances in computational modeling and the integral roles played by artificial intelligence and machine learning. |
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Tuesday February 8 2022 | Special seminar Ingo WALDMANN (UCL)
Deep learning in exoplanet characterisation Summary [click here] The use of machine and deep learning is prevalent in many fields of science and industry and is now becoming more widespread in extrasolar planet and solar system sciences. Deep learning holds many potential advantages when it comes to modelling highly non-linear data, as well as speed improvements when compared to traditional analysis and modelling techniques. However, their often ‘black box’ nature and unintuitive decision processes, are a key hurdle to their broader adoption. In this seminar, I will give an overview of deep learning approaches used in exoplanet characterisation and discuss our recent work on developing Explainable AI (XAI) approaches. XAI is a rapidly developing field in machine learning and aims to make ‘black box’ models interpretable. By understanding how different neural net architectures learn to interpret atmospheric spectra, we can derive more robust prediction uncertainties as well as map information content as function of wavelength. As data and model complexities are bound to increase dramatically with the advent of JWST and ELT measurements, robust and interpretable deep learning models will become valuable tools in our data analysis repertoire. |
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Tuesday November 22 2022 | Recent hiree seminar Antonio GARCIA MUÑOZ (DAp)
Exoplanet atmospheres: An anticipated revolution Summary [click here] The launch of the JWST less than a year ago is expected to set a turning point in exoplanet science, which is progressively transitioning from detection and population statistics to in-detail characterization of the exoplanets’ atmospheres. In this talk, I will present an admittedly biased
perspective of what this may represent for our understanding of exoplanets, and how ongoing theoretical work and future telescopes may build upon JWST’s legacy. |
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Thursday June 29 2023 | Julien DE WIT (Department of Earth, Atmospheric and Planetary Science, MIT, USA)
Robustly Characterizing Exoplanets in a New Era of High-Precision Transit Measurements Summary [click here] With a new-generation of great observatories coming online this decade, unprecedented insights into exoplanets will soon be within reach. Observatories such as the James Webb Space Telescope (JWST) notably enable the study of atmospheres around terrestrial exoplanets and can reveal tri-dimensional structures in the atmospheres of their larger counterparts. Robustly leveraging new observations to reach such achievements will however require extra care as the models currently used may not be up to par with their precision.
During this presentation, I will introduce work done by MIT’s Disruptive Planets group and collaborators towards supporting the robust in-depth characterization of exoplanets. I will specifically discuss how not accounting for the true shape of a planet can lead to a misinterpretation of its interior properties as well as atmospheric structure; how the current state of our understanding of light-matter interactions can similarly affect our interpretation of planetary spectra and thus inferences regarding their atmospheric properties; and how the current state of emission spectrum models for stars may even prevent from disentangling between the contribution of a planet and its host star, to start with. I will also present possible ways to address these challenges. I will end with a step-by-step roadmap to the robust characterization of temperate terrestrial planets with JWST, which includes habitability assessment. |
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Tuesday September 17 2024 | Postdoc seminar Arghyadeep PAUL (LDE3)
Star-Planet Magnetic Interactions: From Solar System to Exoplanets Summary [click here] The interaction between stellar winds and planetary magnetospheres has been a focus of research for decades. Within the heliospheric context, star-planet interactions similar to that between the Sun and the Earth reshapes the planet’s local magnetic environment leading to the formation of magnetospheres. Magnetic reconnection within these magnetospheres gives rise to helical magnetic flux ropes known as flux transfer events (FTEs). We developed a two-way coupled magnetosphere-ionosphere model to investigate the impact of FTEs on the planet’s ionosphere. For a specific case of an Earth like planet, the field aligned currents generated by these FTEs closely resemble observations of discrete dayside auroral arcs, suggesting FTEs to be a probable cause. In the context of exoplanetary systems, star-planet interactions vary depending on the planet's orbital location. For close-in orbits located in the sub-Alfvénic stellar wind, the Poynting flux generated by star-planet interactions can propagate toward the star, giving rise to stellar chromospheric hotspots. Our current work characterises and quantifies the efficiency of this energy transfer
between the planet and the star, revealing that a significant portion of the energy never reaches the star and is reflected by the stellar transition region back toward the planet. The firm detection and characterisation of such magnetic interactions from observations of chromospheric hotspots would also lead to constraints on the amplitude of the magnetic field of exoplanets, to which we are blind so far. Future research, utilising the previously developed magnetosphere-ionosphere model, will also explore how the presence or absence of a planetary ionosphere influences the Poynting flux generated by the planet. |
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Tuesday October 1 2024 | Recent hiree seminar Elsa DUCROT (LDE3)
Detection and characterization of rocky planets around ultra-cool stars Summary [click here] The launch of the James Webb Space Telescope (JWST) marked a significant milestone in the field of exoplanet research. For the first time, we can study the atmospheres of Earth-sized rocky planets—an opportunity that was previously unattainable. However, this is currently possible only around the coolest stars, known as red dwarfs. These stars are ideal targets for several reasons: (1) their smaller size makes it easier to detect and study small, transiting planets; (2) their lower luminosity leads to more frequent planetary transits for the same level of stellar irradiation; (3) they are the most abundant stars in the galaxy, and (4) planets orbiting them are more likely to be rocky planets with short orbital periods. In the near future, only such planetary systems are expected to produce signal-to-noise ratios (SNRs) high enough to confidently confirm or rule out the presence of atmospheres on potentially habitable planets.
In this context, my research focuses on: (1) detecting new rocky, temperate planets around red dwarfs using the SPECULOOS telescopes, and (2) observing these planets with JWST to assess the presence and composition of their atmospheres or determine the nature of their surfaces. In this talk, I will share insights into these areas of my work, including the discovery of the new SPECULOOS-3b exoplanetary system and the detailed characterization of TRAPPIST-1's innermost planets using JWST data. I will also discuss stellar contamination caused by photometric heterogeneities, which represents the primary challenge in characterizing planets around red dwarfs in transit, along with various strategies to overcome this obstacle. Finally, I will offer some perspectives on the future study of rocky planets orbiting red dwarfs. WARNING: the first minutes of the recording are missing. |
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Tuesday November 12 2024 | Joel ONG (Institute for Astronomy, University of Hawaii at Manoa)
New Views into Stellar Rotation from Asteroseismology Summary [click here] Asteroseismology — the analysis and interpretation of stellar oscillations — remains the only means by which we may directly inspect the properties of stellar interiors. I will describe new theoretical developments regarding the interpretation of asteroseismic rotational signatures in a particular class of variable stars — red-giant solar-like oscillators. While waves in most variable stars propagate in a single mode cavity, I formulate a description of multicavity oscillations in these red giants by way of a physical analogy where they behave as "acoustic molecular orbitals". I describe two recent observational results which this formalism has helped us to interpret: 1. the detection of engulfment signatures in evolved red giants, and 2. direct constraints on the misalignment angle between the rotational axis of the core and of the envelope, in the central star of a spin-orbit-misaligned planetary system. |
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Tuesday April 27 2021 | Frédéric BOURNAUD (Departement d’Astrophysique, IRFU, CEA-Saclay et Université Paris-Saclay)
Cosmological simulations of galaxy formation: toward a robust accounting of galaxy populations and star formation across the Universe? Summary [click here] Cosmological simulations of galaxy formation are reaching a high level of accuracy and can finely reproduce some of the main properties of galaxy populations: stellar masses, angular momentum, colors, etc. However, most galaxy formation simulations still fail to account for the detailed structure of galaxies and their global star formation history. Using high-resolution, idealized simulations of galactic dynamics and star formation, I will show that these disagreements are not cosmetic details but point toward a fundamental tension between observations and galaxy formation models. Historically, galaxy formation models predicted galaxies with unrealistically large stellar masses: in modern cosmological simulations, this issue is generally solved though energetic feedback from young stars and supermassive black holes. I will nevertheless show that feedback, as implemented in such simulations, is generally excessive, leading to the early and unrealistic exhaustion of interstellar gas reservoirs. Comparisons to idealized simulations and observations of galactic winds support the conclusion that energetic stellar and black hole feedback cannot be entirely responsible for the regulation of star formation and galaxy growth. Other physical processes likely emerge from sub-galactic scales in the interstellar medium, such as subtle coupling between galactic dynamics and star formation through instabilities and turbulence. Nevertheless, some cosmological simulations can now successfully describe the re-distribution of baryons from galaxies to the intergalactic medium, and I will show that these simulations are a crucial tool not just for galaxy formation but also for modern cosmological surveys. I will finally review how simulations of galaxy formation, evolution and star formation could gain strength in the years to come, in relation with the arrival of exascale supercomputers. |
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Tuesday February 1 2022 | Pascal TREMBLIN (Maison de la Simulation)
Non-ideal self-gravity and cosmology: the importance of correlations in the dynamics of the large-scale structures of the Universe
Summary [click here] Inspired by the statistical mechanics of an ensemble of interacting particles (BBGKY hierarchy), we propose to account for small-scale inhomogeneities in self-gravitating astrophysical fluids by deriving a non-ideal Virial theorem and non-ideal Navier-Stokes equations using a decomposition of the gravitational force into a near- and far-field component. These equations involve the pair radial distribution function (similar to the two-point correlation function), similarly to the interaction energy and equation of state in liquids. Small-scale correlations lead to a non-ideal amplification of the gravitational interaction energy, whose omission leads to a missing mass problem, e.g., in galaxies and galaxy clusters. We also propose an extension of the Friedmann equations in the non-ideal regime. We estimate the non-ideal amplification factor of the gravitational interaction energy of the baryons to lie between 5 and 20, potentially explaining the observed value of the Hubble parameter. Within this framework, the acceleration of the expansion emerges naturally because of the increasing number of sub-structures induced by gravitational collapse, which increases their contribution to the total gravitational energy. A simple estimate predicts a non-ideal deceleration parameter qni~-1; this is potentially the first determination of the observed value based on an intuitively physical argument. We suggest that correlations and gravitational interactions could produce a transition to a viscous regime that can lead to flat rotation curves. This transition could also explain the dichotomy between (Keplerian) LSB elliptical galaxy and (non-Keplerian) spiral galaxy rotation profiles. Overall, our results demonstrate that non-ideal effects induced by inhomogeneities must be taken into account in order to properly determine the gravitational dynamics of galaxies and the larger scale universe. |
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Tuesday May 17 2022 | Recent hiree seminar Benjamin MAGNELLI (DAp)
A decade of the main-sequence of star-forming galaxies: New insights and perspectives on massive galaxy formation Summary [click here] The observed tight correlation between the star formation rate (SFR) and the stellar mass of star-forming galaxies (SFGs) is now well constrained over the last 10 Gyr of look-back time. This so-called main sequence (MS), whose normalization declines from z~3 to 0, is commonly interpreted as evidence that SFGs are evolving primarily through a steady and long star-forming mode, likely sustained by the cold gas accretion along the cosmic web. Over the last decade, a plethora of studies have investigated within this framework the physical properties of SFGs along and across the MS, establishing key scaling relations between, e.g., the stellar mass, gas content, and/or morphology of SFGs in the SFR-stellar mass plane. In this talk I will review past and recent observational evidences of this new MS paradigm and how it has shaped our understanding of the evolution of massive galaxies. Then, I will present the limitations of this simple paradigm, and in particular how it fails to explain the more diverse than anticipated population of MS galaxies (e.g., starburst hidden within the MS), the importance of secondary parameters (e.g., environment) and the transition of SFGs to quiescence. I will conclude by presenting future observational opportunities that can be used to investigate this hidden complexity within the main sequence and to further unveil the physics involved in the evolution of massive galaxies over cosmic time. |
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Tuesday April 11 2023 | Anne VERHAMME (Université de Genève)
Searching for the sources of Cosmic Reionisation Summary [click here] Over the past 5 years, we have made tremendous progress on both direct detections of the escape of ionizing radiation from galaxies, over a broad range of redshifts (and instruments), and the tests and validations of indirect probes of the escape of ionizing radiation from galaxies, both from observations and simulations. I will review these recent achievements, and describe the next steps to understand the nature of the sources of reionisation. |
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Tuesday April 18 2023 | Judith IRWIN (Queen's University, Canada)
CHANG-ES — Past and Future Summary [click here] CHANG-ES (Continuum Halos in Nearby Galaxies — an EVLA Survey) is a project to observe 35 nearby galaxies that are edge-on to the line of sight to focus on their radio halos and the disk-halo connection. Wide-band VLA observations at L-band (1.5 GHz) and C-band (6.0 GHz) have provided opportunities to study in-band spectral indices, and observations in all four Stokes parameters with Rotation Measure Synthesis has led to a new understanding of the structure of kpc-scale magnetic fields in disk galaxies. This talk will highlight some of the results of the project and look to the future, as newly completed S-band (3.0 GHz) observations have filled in the L-band to C-band gap and led to the widest contiguous frequency coverage yet seen for galaxies. |
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Tuesday June 6 2023 | Postdoc seminar Carlos GÓMEZ GUIJARRO (LCEG)
Understanding the role of compact star formation in galaxy evolution with ALMA and JWST Summary [click here] Massive elliptical galaxies in the local universe appear to have their high-redshift analogs in the form of extremely compact quiescent galaxies. Therefore, it seems that compact star formation appears to play a pivotal role in the evolutionary pathways of massive galaxies across cosmic history. However, it remains to be understood what this role is in the broader picture set by the main sequence and the scaling relations in galaxy evolution. From an ALMA survey at 1.1mm, we reveal that compact star formation appears to be the norm in massive star-forming galaxies, and sizes as extended as typical star-forming stellar disks are rare. A population of galaxies with modest star formation rates, but which exhibit extremely compact star formation with starburst-like depletion timescales unveils. Compact star formation appears as a physical driver of depletion timescales, gas fractions, and dust temperatures. Gas and star formation compression seems to be a mechanism that allows to hold their star formation rate even when their gas fractions are low and they are presumably on the way to quiescence. Another population of galaxies missed in the deep optical surveys but bright at far-IR/mm wavelengths unveils thanks to recent JWST observations. We present a study investigating the drivers of dust attenuation in massive galaxies in the JWST-era, showing how the stellar mass and morphology plays an important role, with evidence for more compact stellar profiles resulting in the obscuration of galaxies. |
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Tuesday September 19 2023 | Arjen VAN DER WEL (Ghent University, Belgium)
Galaxy Evolution from z=3 to the Present Day: Recent Insights from JWST and VLT surveys Summary [click here] Thanks to an abundance of excellent observational data the evolution of the population of galaxies in terms of its stellar mass function, the distribution of star-formation across galaxies and their structural properties is known with good precision. The challenge we face, though, is to decipher the evolutionary paths of individual galaxies. Two roads to achieving this goal are 1) to reconstruct star-formation histories of individual galaxies through detailed modeling of high-quality spectra, and 2) attempt to define 'rules' for galaxy evolution that tell us how the collection of individual galaxies must evolve so that the population properties are reproduced. I will describe our recent progress along these lines, enabled by high-quality data from VLT spectroscopy of z~1 galaxies (the LEGA-C survey) and high-quality imaging data from HST (CANDELS) and JWST (CEERS). |
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Tuesday October 3 2023 | Postdoc seminar Lara PANTONI (LFEMI)
Dust millimetre emission in nearby galaxies with NIKA2 (IRAM-30m): major challenges and latest results of the IMEGIN Large Program Summary [click here] The millimetre part of the spectrum is one of the least explored parts of a galaxy’s spectral energy distribution (SED), yet it contains emissions from three fundamentally important physical processes. These processes are thermal emission from dust, free-free emission from ionized gas and synchrotron emission from relativistic charged particles moving in the galactic magnetic field. The NIKA2 camera (IRAM-30m telescope), observing at 1.15 mm and 2 mm, provides additional data points for input into the comprehensive SED models and allows us to:
During my presentation, I will focus on the major challenges linked with data processing, uncertainty propagation, and large-scale emission filtering in NIKA2 maps (due to atmosphere removal during the data reduction process). I will show and discuss the latest significant results on NGC891 (Katsioli et al. 2023); NGC4254 (Pantoni et al. in prep.); NGC2976 and NGC2146 (Ejlali et al. in prep.); millimetre morphology (Nersesian et al. in prep.); future perspectives/applications. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Tuesday November 21 2023 | Recent hiree seminar Camila CORREA (LCEG)
Journey into the Unknown: Dark Matter, Observations of Galaxies, and the Path Forward Summary [click here] The nature of dark matter remains as one of the big unknowns of our time. The general expectation for the coming years is that Euclid, DESI, Rubin LSST, among others, will facilitate the production of wide-field galaxy surveys with exceptionally precise measurements, which will be crucial for unraveling the mysteries surrounding dark matter. We eagerly anticipate that the new observations will reveal deviations from the canonical cold collisionless dark matter paradigm, offering insights into its true nature. However, the key question lingers: will it really happen? What is missing now, that Euclid and others state-of-the-art facilities will change? In this seminar, I will strive to address these questions and review the latest status on dark matter searchers, in order to give you an impression of where we are in the search for dark matter and where we are going.
Throughout the seminar, I will also provide an overview of my work on the nature of dark matter and delve into the pivotal role that cosmological simulations of galaxy formation play in this quest. In the interpretation of data from both current and upcoming state-of-the-art observatories, cosmological simulations have emerged as indispensable tools. Simulations have convinced us of the success of LCDM over large scales, and have given us the capability of breaking the degeneracy driven by baryonic physics and dark matter models. However, can we really trust the outcomes of simulations? with their limited resolution and ad-hoc subgrid prescriptions for galaxy evolution. Maintaining a critical perspective on the observational data we work with and the simulated data we generate daily is crucial for advancing in this field. During the seminar, I will provide updates on the current challenges faced by cosmological simulations and highlight the progress they have achieved. I plan to be as efficient as possible to ensure time for a concluding session that hopefully leaves you with food for thought—an engaging discussion about the future. What steps lie ahead in the development of cosmological simulations? What about on the nature of dark matter? What synergies are needed to be forged between theoretical advancements and observational endeavours? And, importantly, what role will DAp play in this unfolding narrative? Let’s chat more on Tuesday 21 Nov. at 10 am. |
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Tuesday November 28 2023 | Elisa CHISARI (University of Oxford)
A rising tide: galaxy alignments in the Stage IV era Summary [click here] A new generation of galaxy surveys is at our doorstep. By mapping billions of galaxies over the sky, they will deliver precision constraints on the cosmological model. The significant gains in constraining power are accompanied by the need to develop more accurate models of galaxy shapes. Several physical effects will come into play that could be neglected or simplified in previous analyses. I will discuss the constraining power of upcoming surveys, focusing mostly on Rubin Observatory, and the modelling challenges they face. Amongst such challenges, I will focus mostly on the “intrinsic alignments” of galaxies: alignments between their shapes that are likely to arise from tidal interactions between them. I will describe some new modelling strategies that could help reduce their impact on cosmological constraints. Finally, I will present some opportunities that arise for using intrinsic alignments themselves as a probe of cosmology and galaxy evolution. |
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Wednesday December 6 2023 | Ute LISENFELD (University of Granada)
Molecular gas in galaxies in compact groups Summary [click here] Galaxies in compact groups live in an environment where constant interactions take place, both through tidal and through collisional processes. They are are therefore ideal objects to study the influence of the environment on galaxy properties and evolution, in particular the quenching of star formation.
The atomic hydrogen is extremely affected by this environment, with high deficiencies in many cases, whereas the molecular gas content is on average relatively normal in most galaxies. However, the properties of the molecular gas are drastically different if one selects those galaxies, based on their mid-infrared colours from the Spitzer and WISE satallites, that are in transitioning phase from active to quiescent which have have a considerably lower molecular-to-stellar mass and star formation efficiency. In my talk I will present result form observations of the molecular gas and star formation in galaxies in compact groups, both from single dish and interferometric observations, that show indications that in many cases the molecular is highly perturbed (and therefore unable to form stars) in compact groups. |
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Tuesday January 23 2024 | Francisca KEMPER (Institute of Space Sciences (ICE-CSIC) / ICREA / IEEC, Spain)
The interstellar dust reservoir in galaxies Summary [click here] The evolution of interstellar dust reservoirs, and the evolution of galaxies themselves go hand-in-hand, as the presence of dust alters evolutionary drivers, such as the interstellar radiation field and the star formation history, while at the same time, the dust is being formed and altered by processes taking place in galaxies. However, far-infrared and submillimeter studies have revealed enormous dust masses at high redshifts that are difficult to explain with dust production from evolved stars (the so-called "dust budget problem"), while in the nearby universe there is also a significant mismatch between the dust production rate and the dust mass observed in the interstellar medium of galaxies. I will go over some possible explanations in an attempt to find a way forward towards a solution to this seeming discrepancy. |
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Tuesday January 30 2024 | Florian PACAUD (University of Bonn)
Galaxy clusters and filaments in the first eROSITA all-Sky survey Summary [click here] Launched in July 2019, eROSITA was set to survey the sky in the soft X-ray band for four years and provide a much awaited update to the old ROSAT all-sky survey. In a few weeks, the collaboration will release the first all-sky map obtained from the first six month of observation. In this talk, I will present the instrument and its technical abilities. I will then explore the all-sky map and show selected results on individual local clusters and filaments. Finally, I will give a preview of the soon-to-be-released data products, including AGN and galaxy catalogs, and present some of the methods used for their cosmological analysis. |
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Tuesday February 27 2024 | Hakim ATEK (IAP)
Unveiling the faintest and the brightest galaxies at early times with JWST Summary [click here] The JWST is revolutionizing our understanding of the early Universe by unveiling a wealth of bright galaxies at z>9 and faint AGNs at z>5. I will present the latest constraints on the overabundance of UV-bright galaxies at z>9, which is 10-100 times higher than galaxy formation models. I will discuss to what extent recent theoretical efforts can reproduce such observations, and how future wide-area surveys such as Euclid will help put stronger constraints ion the bright-end of UVLF at z>8. On the other hand, faint galaxies, representing the building blocks of present-day galaxies, have eluded spectroscopic constraints, even with the deepest JWST campaigns so far. I will present the results of our UNCOVER survey, which combines ultra-deep NIRSpec spectroscopy with the strong lensing magnification of A2744 cluster. We characterize ultra-faint galaxies with intrinsic absolute magnitude between Muv=-17 and Muv=-15 at 6<z<8, and stellar masses down to 10^6 solar masses. I will discuss our plans to obtain the deepest observations on sky with the GLIMPSE program to the faintest galaxy population out to z=15 and beyond. |
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Tuesday March 12 2024 | Postdoc seminar Arturo NUNEZ (LMPA)
The impact of baryonic physics in galaxy formation: Multi-scale approach to galaxy simulations, from the initial mass function to galaxies in a cosmological environment Summary [click here] The interplay of baryonic physics (star formation and feedback processes) in shaping galaxies and their host halos is a multiscale and multidisciplinary problem. Numerical simulations are key to understanding these processes as they cover phenomena that take place at a wide range of scales, some too small for most astrophysical observations but yet able to impact the shape and evolution of galaxies as a whole. However, no single simulation can address every aspect of this complex issue. A comprehensive approach is essential to integrate learnings from various types of simulations and compare them with observations. This includes understanding star formation histories in cosmological simulations, examining interstellar medium dynamics in comparison to simulations of individual galaxies, and exploring the non-universality in the stellar initial mass function in specific molecular cloud simulations. My goal is to discuss how simulations across different scales, together with detailed multiscale observations, collectively contribute to approaching the complexities of galaxy formation and evolution. All this while discussing current shortcomings and successes of high-resolution numerical simulations of galaxies and galactic environments.
WARNING: incomplete video recording. |
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Tuesday April 2 2024 | James DUNLOP (University of Edinburgh)
Charting the emergence of the first galaxies with JWST Summary [click here] The James Webb Space Telescope (JWST) is transforming our view of galaxy formation and evolution in the young Universe. I will provide an overview of the latest results from the PRIMER survey, the largest JWST Cycle-1 “Galaxies” programme which, in combination with other public JWST imaging, is now enabling us to chart the early growth of the galaxy population back to within ~300 million years of the Big Bang. Specifically, I will present and discuss the first robust determination of the evolving galaxy luminosity function extending out to redshifts z~13, as well as new measurements of the galaxy stellar mass function reaching out to z~9. I will then attempt to interpret these results in the context of our current understanding of the evolving dark matter halo mass function and the efficiency with which galaxies are able to convert their baryons into stars. Finally, I will discuss the prospects for future progress with JWST and with the Atacama Large Millimeter Array (ALMA), including the possibility of uncovering the emergence of the first galaxies. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Tuesday November 23 2021 | Special seminar Michel TOULMONDE (SYRTE, Observatoire de Paris)
Émilie du Châtelet et les Principia de Newton Summary [click here] Selon Newton, la cause des mouvements des planètes et des comètes est la gravitation universelle qui crée les forces nécessaires pour que ces astres se déplacent sur des orbites elliptiques autour du Soleil. Il publie ce travail en 1687 dans sa grande œuvre les Philosophiæ naturalis principia mathematica, communément appelée les Principia.
Très admirative de ces idées nouvelles, Émilie Du Châtelet traduit en français le texte latin des Principia dans le but de les diffuser. Son action n'est pas solitaire : Maupertuis, Voltaire et Clairaut notamment vont la soutenir dans sa lourde tâche, car plusieurs savants français n’acceptent pas l’idée d’action à distance que serait la gravitation. Après le décès d'Émilie en 1749, Clairaut devient l'éditeur du manuscrit des Principes mathématiques de la Philosophie naturelle, mais l’ouvrage ne sera publié que dix ans plus tard, en 1759. À l’occasion de la parution d’une nouvelle édition critique de ce livre, on reverra les apports fondamentaux de Newton, puis on regardera le manuscrit d’Émilie Du Châtelet et on verra le rôle de Clairaut dans la parution tardive des Principes et du Commentaire écrits par « l’immortelle Émilie ». |
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Tuesday November 8 2022 | Philippe Laurent, Aymeric Sauvageon, Jérôme Rodriguez (DAp)
INTEGRAL : 20 ans dans l'espace pour une aventure de plus de 35 ans Summary [click here] Le laboratoire INTErnational d'Astrophysique des Rayons Gamma (INTEGRAL) a été lancé le 17 octobre 2002 de Baikonour (Kasakstan). Depuis lors il est resté sur son orbite elliptique de haute excentricité (environ 3 jours) effectuant 2568 révolutions (au 8 novembre 2022) autour de la Terre pour environ 530 Ms (méga secondes) d'observations scientifiques. Cette mission de taille moyenne transporte deux instruments principaux opérant dans le domaine spectral des rayons X durs et des rayons gamma mous (20keV-10 MeV), dont l'optique est basée sur le concept de masques codés : l'un est dédié à l'analyse spectrale fine avec des capacités d'imagerie modérées (SPI), l'autre est dédié à l'imagerie fine (ou presque) avec des capacités spectrales modérées (IBIS).
Ces 20 ans dans l'espace sont le résultat d'au moins 15 ans de développement du projet avant qu'INTEGRAL puisse dévoiler le mystère du ciel à haute énergie. Le Dap a été profondément impliqué dans cette aventure de longue haleine, depuis le tout début du développement du concept de la mission (grande implication dans les deux instruments, ISGRI et SPI, participation au centre de données, développement de l'analyse s/w, suivi de la caméra) jusqu'à l'analyse actuelle des données en temps réel et des archives, avec un grand nombre d'articles publiés dans tous les domaines permis par les capacités instrumentales et même au-delà. Nous allons, dans ce séminaire, résumer ces plus de 35 ans en présentant divers aspects du projet/de la mission, en nous concentrant particulièrement sur ceux où l'implication du Dap a été cruciale. Nous commencerons par une vue d'ensemble de l'histoire de la mission, des principales caractéristiques instrumentales et des objectifs scientifiques au lancement, puis nous nous concentrerons sur la caméra ISGRI, la couche de détection 20-250 keV du télescope IBIS, qui est suivie au Dap tous les jours. ISGRI a été développée à l'IRFU et a obtenu la plupart des résultats d'INTEGRAL. Nous conclurons par un aperçu rapide et évidemment biaisé de quelques résultats scientifiques obtenus au cours de ces 20 années. Le séminaire sera présenté en Français avec des diapositives en Anglais par Philippe Laurent, Aymeric Sauvageon et Jérôme Rodriguez, bien humblement au nom d'un grand nombre de collègues anciens et actuels. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH VERSION INTEGRAL : 20 years in space and for a 35+ years adventure The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) has been launch on October 17th, 2002 from Baikonour (Kasakstan). Since then it has remained on its high-eccentricity elliptical orbit (about 3 days) performing 2568 revolutions (as of Nov. 8th, 2022) around the Earth for around 530 megaseconds of scientific observations. This medium sized mission caries two main instruments operating in the spectral domain of hard X-rays/Soft Gamma-rays (20keV-10 MeV), whose optics is based on the concept of coded masks: one is dedicated to fine spectral analysis with moderate imaging capabilities (SPI) the other is dedicated to fine(-ish) imaging with moderate spectral capabilities (IBIS). These 20 years in space are the results of at least 15 years of project development before INTEGRAL could unveil the mystery of the high-energy sky. The DAp has been deeply involved in this long-term adventure from the very beginning of the mission concept development (large involvement in both instruments, ISGRI and SPI, participation to the data centre, development of the s/w analysis, monitoring of the camera) to the current analysis of real time and archival data, with a large number of published papers in all fields allowed by the instrumental capabilities and even beyond. We will, in this seminary, summarised these 35 years+ by presenting various aspects of the project/mission, focusing especially on those where the involvement of the Dap has been crucial. This will start with an overall overview of the mission history, the main instrumental characteristics and scientific goals at launch, followed by a focus on the ISGRI camera, the 20-250 keV detector layer of the IBIS telescope, which is followed-up at DAp every days. ISGRI has been developed at IRFU and obtained most of the INTEGRAL results. We will conclude with a quick and obviously biased overview of some scientific results obtained over these 20 years. The talks will be given by Philippe Laurent, Aymeric Sauvageon, and Jérôme Rodriguez humbly on behalf of many, many former and current colleagues and presented in French with slides in English. |
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Friday November 25 2022 | Special seminar Leïla GODINAUD, Anne DECOURCHELLE, Fabio ACERO (DAp)
De Nova Stella : Séminaire d'anniversaire des 450 ans de la supernova de Tycho Summary [click here] En novembre 1572, l'observation de l'apparition d'une "nouvelle étoile" dans le ciel et les observations détaillées réalisées par Tycho Brahe ont transformé la façon dont nous voyons et comprenons notre Univers. Pour marquer l'événement, nous proposons un séminaire en trois parties allant de l'observation historique de l'événement jusqu'aux connaissances actuelles sur la supernova SN1572 et son vestige.
La conférence sera suivie d'un café gourmand dans le hall de la salle Galilée. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH De Nova Stella: 450th anniversary seminar for Tycho's supernova In November 1572, the observation of the apparition of a "new star" in the sky and the detailed observations made by Tycho Brahe transformed the way we see and understand our Universe. To mark the event we propose a three-part seminar ranging from the historical observation of the event to the current knowledge about the supernova SN1572 and its remnant. The conference will be followed by a gourmet coffee in the hall of the Galilee room. |
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Tuesday January 10 2023 | Stéphane AUNE, Jean-Charles CUILLANDRE et Pierre ASTIER (CEA/SEDI, CEA/DAp et LPNHE)
20 years of MegaCam scientific operations on the Canada-France-Hawaii Telescope Summary [click here] Over its history CFHT has helped build or hosted dozens of instruments, operating from ultraviolet to mid-infrared wavelengths, attuned to Maunakea’s renowned observing conditions. One instrument, though, eclipses them all – MegaCam. It is in fact hard to imagine CFHT’s success today without the decisions made two decades ago to build what was then the largest digital focal plane in the world. MegaCam took engineering techniques to closely mount CCDs into a single camera to a whole new level. With its 1 degree field of view, MegaCam was able to tile the sky with images, creating rich and precise datasets on enormous scales that could be mined by the entire scientific community for decades. To date MegaCam has led to more peer reviewed science publications than all other instruments used at CFHT combined. It has been used to tune the model of Dark Energy and map vast regions of Dark Matter, unveil faint wisps of stars flung from interacting galaxies, reveal hundreds of free floating planets in star forming regions, and is currently directly contributing to a Stage IV dark energy measurement by providing the key ground-based photometry for the ESA Euclid space mission to derive photometric redshifts of hundreds of millions of galaxies over the northern sky. All of this is part of a revolution in wide field high-resolution optical imaging that CFHT pioneered for many years. Three speakers will offer a complete overview from technical aspects to the most impactful scientific result over these past two decades:
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Tuesday November 5 2024 | Special seminar Jean-Marc BONNET-BIDAUD (DAp)
Chine, la civilisation du ciel transitoire Summary [click here] En Chine, dès l'époque classique des Han (IIe siècle AEC), le Ciel est conçu comme le miroir de la Terre, lieux d'évènements fortuits (éclipses, comètes, taches solaires, novae et supernovae,..) qui sont traqués et catalogués par une armée de scientifiques au service de l'empereur. De nombreux documents astronomiques ont ainsi été produits sur plusieurs millénaires, conservés ou récemment redécouverts par l’archéologie. C’est le cas notamment de la plus ancienne carte d’étoiles connue au Monde conçue il y a plus de 1200 et préservée dans un monastère bouddhique de la route de la Soie.
Avec la présentation de ces documents, on retracera les étapes, les instruments et les concepts qui ont mené l'astronomie chinoise ancienne à de nombreuses découvertes du ciel transitoire, à des époques où l’Europe n’envisageait encore le Ciel que comme une voute céleste fixe, éternelle et immuable. Avec ce recul de l’histoire, la mission franco-chinoise SVOM, qui va détecter et cataloguer les plus puissants phénomènes célestes variables, apparait aujourd'hui comme une continuation logique dans laquelle la France rejoint symboliquement la Chine dans la préoccupation du ciel transitoire. |
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Tuesday November 29 2022 | Elisa COSTANTINI (SRON, Netherlands)
The cold and dusty medium in our Galaxy as seen in the X-rays Summary [click here] X-rays have unique advantages in studying absorption and scattering from interstellar dust. For example, sharp and deep absorption features of Mg, Si, O and Fe, which are the building blocks of silicates, fall in the X-ray band. Present X-ray observatories already delivered to us interesting results, challenging the common paradigm on interstellar dust chemical and physical characteristics. Future instruments will open up an unexplored window, revealing the most dense environments of our Galaxy.
In this talk I will illustrate the state-of-art of our understanding of dust as seen in the X-rays, and future prospects, using for example, the upcoming XRISM satellite. |
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Tuesday October 3 2023 | Postdoc seminar Lara PANTONI (LFEMI)
Dust millimetre emission in nearby galaxies with NIKA2 (IRAM-30m): major challenges and latest results of the IMEGIN Large Program Summary [click here] The millimetre part of the spectrum is one of the least explored parts of a galaxy’s spectral energy distribution (SED), yet it contains emissions from three fundamentally important physical processes. These processes are thermal emission from dust, free-free emission from ionized gas and synchrotron emission from relativistic charged particles moving in the galactic magnetic field. The NIKA2 camera (IRAM-30m telescope), observing at 1.15 mm and 2 mm, provides additional data points for input into the comprehensive SED models and allows us to:
During my presentation, I will focus on the major challenges linked with data processing, uncertainty propagation, and large-scale emission filtering in NIKA2 maps (due to atmosphere removal during the data reduction process). I will show and discuss the latest significant results on NGC891 (Katsioli et al. 2023); NGC4254 (Pantoni et al. in prep.); NGC2976 and NGC2146 (Ejlali et al. in prep.); millimetre morphology (Nersesian et al. in prep.); future perspectives/applications. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Wednesday December 6 2023 | Ute LISENFELD (University of Granada)
Molecular gas in galaxies in compact groups Summary [click here] Galaxies in compact groups live in an environment where constant interactions take place, both through tidal and through collisional processes. They are are therefore ideal objects to study the influence of the environment on galaxy properties and evolution, in particular the quenching of star formation.
The atomic hydrogen is extremely affected by this environment, with high deficiencies in many cases, whereas the molecular gas content is on average relatively normal in most galaxies. However, the properties of the molecular gas are drastically different if one selects those galaxies, based on their mid-infrared colours from the Spitzer and WISE satallites, that are in transitioning phase from active to quiescent which have have a considerably lower molecular-to-stellar mass and star formation efficiency. In my talk I will present result form observations of the molecular gas and star formation in galaxies in compact groups, both from single dish and interferometric observations, that show indications that in many cases the molecular is highly perturbed (and therefore unable to form stars) in compact groups. |
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Tuesday January 23 2024 | Francisca KEMPER (Institute of Space Sciences (ICE-CSIC) / ICREA / IEEC, Spain)
The interstellar dust reservoir in galaxies Summary [click here] The evolution of interstellar dust reservoirs, and the evolution of galaxies themselves go hand-in-hand, as the presence of dust alters evolutionary drivers, such as the interstellar radiation field and the star formation history, while at the same time, the dust is being formed and altered by processes taking place in galaxies. However, far-infrared and submillimeter studies have revealed enormous dust masses at high redshifts that are difficult to explain with dust production from evolved stars (the so-called "dust budget problem"), while in the nearby universe there is also a significant mismatch between the dust production rate and the dust mass observed in the interstellar medium of galaxies. I will go over some possible explanations in an attempt to find a way forward towards a solution to this seeming discrepancy. |
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Tuesday February 6 2024 | Stefanie WALCH-GASSNER (University of Köln)
The impact of stellar feedback on different scales and galactic environments Summary [click here] Stellar feedback refers to the processes by which massive stars release energy, radiation and material into their surroundings, influencing the structure and evolution of the galaxies in which they reside. Understanding the impact of stellar feedback on different galactic environments is crucial for developing a comprehensive picture of galaxy formation and evolution. In this context, different galactic environments refer to regions within a galaxy that differ in their physical conditions, such as the average gas density, temperature, or metallicity.
We study the respective impact of stellar winds, ionizing radiation, and supernovae in modern simulations of the multi-phase interstellar medium in parts of galaxies within the SILCC project, which I will present in this talk. From these galactic scale simulations we find that ionizing radiation is the most important factor in regulating the star formation rate, while supernova over-pressure the gas substantially, thus driving a galactic outflow. |
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Tuesday February 13 2024 | Jonathan TAN (Chalmers & University of Virginia)
A Light in the Dark - Massive Star Birth Through Cosmic Time Summary [click here] Massive stars are important thoughout the universe, but their formation remains poorly understood. I review current understanding of how massive stars and star clusters form in our Galaxy, including models for how star formation is triggered in giant molecular clouds and tests of how individual massive stars form from smaller scale clumps and cores. Finally, I discuss how massive star formation may have been different in the very early universe and how the first stars may have seeded the supermassive black holes powering active galactic nuclei. |
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Tuesday May 7 2024 | Thomas HENNING (Max Planck Institute for Astronomy, Heidelberg)
Towards New Horizons: Star and Planet Formation with JWST Summary [click here] The James Webb Space Telescope (JWST) is the most powerful space-based infrared observatory ever built. The first part of the talk will discuss the main structures of the telescope and its assembly after launch. The talk will then highlight the extremely exciting science results in the field of star and planet formation, including the nature of protostars and planet-forming disks. A special emphasis will be placed on the accretion process and the chemical building blocks of disks and what we can learn from infrared spectroscopy regarding the inventory of material in the terrestrial planet-forming zone. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday March 22 2022 | Benjamin WEHMEYER (CSFK, Budapest)
Galactic Chemical Evolution of rapid neutron capture process elements using special, rare classes of supernovae, and of short lived radioisotopes Summary [click here] The origin of the heaviest elements is still a matter of debate. For the rapid neutron capture process (r-process), multiple sites have been proposed, e.g., neutron star mergers and (sub-classes) of supernovae (e.g., magnetorotationally driven supernovae). R-process elements have been measured in a large fraction of metal-poor stars. Galactic archaeology studies show that the r-process abundances among these stars vary by over 2 orders of magnitude. On the other hand, abundances in stars with solar-like metallicity do not differ greatly. This leads to two major open questions:
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Friday November 25 2022 | Special seminar Leïla GODINAUD, Anne DECOURCHELLE, Fabio ACERO (DAp)
De Nova Stella : Séminaire d'anniversaire des 450 ans de la supernova de Tycho Summary [click here] En novembre 1572, l'observation de l'apparition d'une "nouvelle étoile" dans le ciel et les observations détaillées réalisées par Tycho Brahe ont transformé la façon dont nous voyons et comprenons notre Univers. Pour marquer l'événement, nous proposons un séminaire en trois parties allant de l'observation historique de l'événement jusqu'aux connaissances actuelles sur la supernova SN1572 et son vestige.
La conférence sera suivie d'un café gourmand dans le hall de la salle Galilée. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH De Nova Stella: 450th anniversary seminar for Tycho's supernova In November 1572, the observation of the apparition of a "new star" in the sky and the detailed observations made by Tycho Brahe transformed the way we see and understand our Universe. To mark the event we propose a three-part seminar ranging from the historical observation of the event to the current knowledge about the supernova SN1572 and its remnant. The conference will be followed by a gourmet coffee in the hall of the Galilee room. |
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Tuesday November 29 2022 | Elisa COSTANTINI (SRON, Netherlands)
The cold and dusty medium in our Galaxy as seen in the X-rays Summary [click here] X-rays have unique advantages in studying absorption and scattering from interstellar dust. For example, sharp and deep absorption features of Mg, Si, O and Fe, which are the building blocks of silicates, fall in the X-ray band. Present X-ray observatories already delivered to us interesting results, challenging the common paradigm on interstellar dust chemical and physical characteristics. Future instruments will open up an unexplored window, revealing the most dense environments of our Galaxy.
In this talk I will illustrate the state-of-art of our understanding of dust as seen in the X-rays, and future prospects, using for example, the upcoming XRISM satellite. |
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Tuesday September 12 2023 | Alexander MOISEEV (NASA/GSFC and University of Maryland)
New Mission Concept: Compton Telescope with Coded Aperture Mask, and its Science Perspectives Summary [click here] The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel Explorer-class concept for a next-generation telescope covering the poorly explored hard X-ray and soft gamma-ray energies. This concept builds upon the heritage of past and current missions, improving sensitivity and, very importantly, angular resolution. GECCO uses the combined Coded Aperture Mask and Compton telescope techniques to employ the benefits of both: superior angular resolution provided by the deployable Coded Aperture Mask, and good background rejection and wide field-of-view (FoV) provided by the Compton telescope. It is being developed at NASA/GSFC in collaboration with other US and foreign institutions. GECCO observations will extend arcminute angular resolution to high-energy images of the Galactic plane, combining the
spectral capabilities of INTEGRAL/IBIS and the x-ray imaging of NuSTAR and eROSITA, and will make a bridge to the Fermi-LAT observations, enabling a broad potential for discoveries in the MeV γ-ray sky.
With the unprecedented angular resolution of the coded mask telescope combined with the sensitive, wide FoV Compton telescope, GECCO will focus on two main science objectives:
The GECCO design is based on the novel CZT Imaging calorimeter, which serves as a standalone Compton telescope and as a focal plane detector for the Coded Mask. It also is a powerful tool to measure the γ-radiation polarization. GECCO’s octagon-shaped active shield also serves as a powerful all-sky detector of gamma-ray bursts, prompting the instrument to slew towards the burst direction and localize it with the Coded Aperture’s arcminute accuracy. GECCO will operate in the 100 keV - 10 MeV energy range, with energy resolution of ~ 1% from 0.5 - 5 MeV. The Coded Aperture Mask provides the angular resolution of ∼1 arcmin with a 2 × 2 deg2 fully coded field-of-view, while the Compton telescope provides the angular resolution of 3◦ − 6◦ with a 60 × 60 deg2 field-of-view. The 3σ, 106 s sensitivity is expected to be about 10-5 MeV cm-2 s-1 over the entire energy range. Primary mode of observation is fixed pointing, with extended exposure of the regions of interest. However, as a standalone Compton telescope with wide FoV, Imaging Calorimeter will simultaneously provide wide-area sky exploration, significantly broadening GECCO’s observational scope. |
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Tuesday January 23 2024 | Francisca KEMPER (Institute of Space Sciences (ICE-CSIC) / ICREA / IEEC, Spain)
The interstellar dust reservoir in galaxies Summary [click here] The evolution of interstellar dust reservoirs, and the evolution of galaxies themselves go hand-in-hand, as the presence of dust alters evolutionary drivers, such as the interstellar radiation field and the star formation history, while at the same time, the dust is being formed and altered by processes taking place in galaxies. However, far-infrared and submillimeter studies have revealed enormous dust masses at high redshifts that are difficult to explain with dust production from evolved stars (the so-called "dust budget problem"), while in the nearby universe there is also a significant mismatch between the dust production rate and the dust mass observed in the interstellar medium of galaxies. I will go over some possible explanations in an attempt to find a way forward towards a solution to this seeming discrepancy. |
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Tuesday February 13 2024 | Jonathan TAN (Chalmers & University of Virginia)
A Light in the Dark - Massive Star Birth Through Cosmic Time Summary [click here] Massive stars are important thoughout the universe, but their formation remains poorly understood. I review current understanding of how massive stars and star clusters form in our Galaxy, including models for how star formation is triggered in giant molecular clouds and tests of how individual massive stars form from smaller scale clumps and cores. Finally, I discuss how massive star formation may have been different in the very early universe and how the first stars may have seeded the supermassive black holes powering active galactic nuclei. |
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Tuesday May 7 2024 | Thomas HENNING (Max Planck Institute for Astronomy, Heidelberg)
Towards New Horizons: Star and Planet Formation with JWST Summary [click here] The James Webb Space Telescope (JWST) is the most powerful space-based infrared observatory ever built. The first part of the talk will discuss the main structures of the telescope and its assembly after launch. The talk will then highlight the extremely exciting science results in the field of star and planet formation, including the nature of protostars and planet-forming disks. A special emphasis will be placed on the accretion process and the chemical building blocks of disks and what we can learn from infrared spectroscopy regarding the inventory of material in the terrestrial planet-forming zone. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday March 14 2023 | Special seminar Marie GUÉGUEN (Université de Rennes)
Cosmology: a tension within code comparisons Summary [click here] Code comparisons in cosmology are often performed with the underlying objective of identifying predictions upon which different codes converge that can be interpreted as robust, free of artifacts, predictions. Such an objective assumes that code comparisons can be constructed in such a way that the two notions of convergence and of robustness collapse. But, in order to achieve such a goal, code comparisons of structure formation have to meet an important epistemic challenge: that of constructing their codes ensemble on the basis of codes that are as independent as possible, but also comparable. In this talk, I show that enforcing the latter often amounts to multiplying common idealizations that hinder the achievement of the former, and thus leads to code comparisons that include many unscrutinized sources of artefacts possibly similarly distorting the predictions of the model. As a result, I argue that in context of high uncertainties where the domain of verification and validation of simulations has shrunk to code comparisons, code comparisons are better and actually very efficient as exploratory tools, both for getting insights into the physics implemented and for breaking the epistemic opacity of numerical simulations. |
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Tuesday November 5 2024 | Special seminar Jean-Marc BONNET-BIDAUD (DAp)
Chine, la civilisation du ciel transitoire Summary [click here] En Chine, dès l'époque classique des Han (IIe siècle AEC), le Ciel est conçu comme le miroir de la Terre, lieux d'évènements fortuits (éclipses, comètes, taches solaires, novae et supernovae,..) qui sont traqués et catalogués par une armée de scientifiques au service de l'empereur. De nombreux documents astronomiques ont ainsi été produits sur plusieurs millénaires, conservés ou récemment redécouverts par l’archéologie. C’est le cas notamment de la plus ancienne carte d’étoiles connue au Monde conçue il y a plus de 1200 et préservée dans un monastère bouddhique de la route de la Soie.
Avec la présentation de ces documents, on retracera les étapes, les instruments et les concepts qui ont mené l'astronomie chinoise ancienne à de nombreuses découvertes du ciel transitoire, à des époques où l’Europe n’envisageait encore le Ciel que comme une voute céleste fixe, éternelle et immuable. Avec ce recul de l’histoire, la mission franco-chinoise SVOM, qui va détecter et cataloguer les plus puissants phénomènes célestes variables, apparait aujourd'hui comme une continuation logique dans laquelle la France rejoint symboliquement la Chine dans la préoccupation du ciel transitoire. |
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Tuesday June 27 2023 | Julien AUBERT (IPGP)
Variations géomagnétiques rapides: un nouveau message émis par le noyau de la Terre Summary [click here] Le signal géomagnétique est une source riche d’informations sur la structure, la dynamique interne et l’histoire de notre planète. La production du champ magnétique Terrestre par effet dynamo dans le noyau externe implique une large disparité d’échelle spatiales et plus spécifiquement temporelles, s’étalant sur une gamme allant de l’année au milliard d’années. Depuis la mise en place d’observatoires magnétiques à la surface émergée de la Terre, l’attention s’est portée sur l’explication des variations à l’échelle du siècle, qui sont liées aux mouvements de convection dans le noyau. Depuis une vingtaine d’années, une couverture satellitaire globale et continue a cependant mis en évidence des variations de l’année à la dizaine d’années, dont l’origine est débattue. Ces nouvelles données ouvrent une fenêtre sur des phénomènes magnétohydrodynamiques rapides, de nature ondulatoire, en interaction avec la convection lente dans le noyau. Dans cet exposé, je présenterai les défis posés par la simulation conjointe des deux phénomènes ainsi que les avancées récentes et applications géophysiques potentielles. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Tuesday June 11 2024 | Mustapha MEFTAH (LATMOS)
Les NanoSatellites au service de l’observation de la Terre et de l’univers Summary [click here] Le thème de la conférence portera sur l’observation de la Terre, le climat, les satellites et le NewSpace. Le 24 janvier 2021, une fusée Falcon 9 était lancée avec succès par SpaceX depuis Cap Canaveral, en Floride. Sous la coiffe du lanceur, il y avait 143 satellites. C’est à ce jour le plus grand nombre de satellites jamais déployés en une seule mission. Uvsq-Sat, premier nanosatellite français dédié à l’observation de variables climatiques essentielles, faisait partie de cette épopée. Depuis plus de trois ans, Uvsq-Sat observe la Terre et le Soleil de manière assidue. A 550 km d’altitude, ce satellite mesure la quantité d'énergie solaire réfléchie vers l'espace et la quantité d'énergie thermique émise par la Terre vers l’espace.
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Tuesday September 17 2024 | Postdoc seminar Arghyadeep PAUL (LDE3)
Star-Planet Magnetic Interactions: From Solar System to Exoplanets Summary [click here] The interaction between stellar winds and planetary magnetospheres has been a focus of research for decades. Within the heliospheric context, star-planet interactions similar to that between the Sun and the Earth reshapes the planet’s local magnetic environment leading to the formation of magnetospheres. Magnetic reconnection within these magnetospheres gives rise to helical magnetic flux ropes known as flux transfer events (FTEs). We developed a two-way coupled magnetosphere-ionosphere model to investigate the impact of FTEs on the planet’s ionosphere. For a specific case of an Earth like planet, the field aligned currents generated by these FTEs closely resemble observations of discrete dayside auroral arcs, suggesting FTEs to be a probable cause. In the context of exoplanetary systems, star-planet interactions vary depending on the planet's orbital location. For close-in orbits located in the sub-Alfvénic stellar wind, the Poynting flux generated by star-planet interactions can propagate toward the star, giving rise to stellar chromospheric hotspots. Our current work characterises and quantifies the efficiency of this energy transfer
between the planet and the star, revealing that a significant portion of the energy never reaches the star and is reflected by the stellar transition region back toward the planet. The firm detection and characterisation of such magnetic interactions from observations of chromospheric hotspots would also lead to constraints on the amplitude of the magnetic field of exoplanets, to which we are blind so far. Future research, utilising the previously developed magnetosphere-ionosphere model, will also explore how the presence or absence of a planetary ionosphere influences the Poynting flux generated by the planet. |
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Tuesday September 13 2022 | Special seminar Elisabeth KOHLER (CNRS)
Agir pour l’égalité professionnelle entre les femmes et les hommes Summary [click here] De la scolarité aux postes à responsabilité de nombreux stéréotypes persistent qui souvent entravent la carrière scientifique des femmes. L’objectif de ce séminaire est d’identifier ces biais et de présenter des exemples de mesures concrètes pour y remédier et mettre en œuvre des politiques d’égalité professionnelle entre les femmes et les hommes. Différents sujets seront abordés, tels que les critères d’évaluation, les actions de communication, le congé maternité et la parentalité, la lutte et la prévention contre les violences sexistes et sexuelles. |
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Tuesday May 16 2023 | Special seminar Vianney LEBOUTEILLER (LFEMI)
Soft-Skilling in Fundamental Research Summary [click here] Artificial intelligence (AI) holds more and more importance in our lives and in our work. While AI undeniably provides a smart and useful companion, the current debates of whether it may replace us altogether in our tasks highlights the role and importance of social interactions and emotions in the work environment at large.
In this non-astrophysics seminar, I will present a recent study whose goal is to identify socio-professional (a.k.a. transversal, transferable, or generally "soft") skills that are critical in the transformation and innovation of companies/industries. As it turns out, most of these skills are actually acquired or at least strengthened throughout a career in fundamental research, including during the PhD. This is not surprising as transformation, innovation, but also breakthroughs, creativity etc... are common motivations in both worlds in which we seek answers to questions but we also seek questions themselves, within a complex environment and network of people. Many difficulties arise when dealing with soft skills, however: identifying them, acquiring and/or realizing they have been acquired, improving them, measuring them, making them valuable, and convincing other people they have been acquired. All in all, while most soft skills are well-known, the process of sorting, grouping, ranking them is necessary to set reference frameworks that can be acknowledged by most people. The purpose of this talk is to describe what soft skills are, how they are an integral part of research, and preliminary thoughts on how they can be applied/converted to non-academic world. |
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Tuesday May 21 2024 | Climate seminar Céline GUIVARCH (École des Ponts ParisTech)
Le changement climatique vu à travers les lunettes d’une économiste Summary [click here] Les effets du changement climatique constituent une menace grandissante pour nos sociétés et nos économies; et agir face à cette menace nécessite de transformer nos façons de nous déplacer, nous loger, de produire et consommer. Autant de questions pour l'économie, pour analyser les implications économiques des trajectoires et politiques publiques d'atténuation du changement climatique. Ce séminaire proposera quelques incursions dans la recherche actuelle en économie du changement climatique, à partir d'un échantillon d'articles récents.
Le séminaire sera en français avec des slides en anglais. |
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Tuesday October 15 2024 | Climate seminar Sophie SCHBATH (INRAE)
Parcours du laboratoire MaIAGE pour réduire l'empreinte environnementale de ses activités Summary [click here] L'unité MaIAGE a commencé à réfléchir et adopter des éco-gestes dès 2017. En 2020, elle réalise son premier bilan de gaz à effet de serre (GES) sur les données de 2019 et se porte volontaire en 2021 pour participer à l'expérimentation nationale du GDR Labo 1point5. Cette expérimentation, conduite sur 22 labos pilotes, vise à étudier la mise en mouvement des laboratoires vers une réduction de leurs émissions de GES et à explorer différents dispositifs de réduction. C'est dans ce cadre que l'unité a voté en 2022 un scénario visant 40% de réduction de ses GES en 2030 ; ce scénario s'accompagne d'un certain nombre de mesures obligatoires et/ou incitatives. C'est l'ensemble de ce parcours qui sera présenté lors de la présentation avec des exemples d'actions réalisées et de premiers résultats. |
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Tuesday April 19 2022 | Sacha BRUN et Olivier LIMOUSIN (DAp)
Solar Orbiter: the heliospheric explorer Summary [click here] Solar Orbiter - ESA M1 Mission - has entered its scientific phase at the end of November 2021, after a cruise phase of more than 18 months (and 2.2 Billion km) and the commissioning of the 10 instruments on board. CEA/IRFU played a key role in this mission, by providing the focal plane detector array of the X-ray telescope, STIX, based on in-house Caliste technology. In the meantime, solar physicists have been busy preparing the pipelines needed to handle the data sent by the instrument, and developing high performance numerical simulations of the Sun.
In this 2-voice seminar, we will relate the first 26 months of activities of the mission, covering both STIX calibration, its first light, the first solar flare detected, as well as the development of associated numerical simulations of the Sun and optimal scientific processing of instrumental data sent as the spacecraft gets closer and closer to the Sun (last perihelion was on 26 March 2022 at 0.32 AU), while in the meantime our star is increasing in intensity and in activity, with solar magnetic cycle 25 now well on its way. |
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Tuesday September 26 2023 | Postdoc seminar Adam FINLEY (LDE3)
Studying the whole Sun: from small-scale heating to large-scale dynamics Summary [click here] The Sun’s large-scale magnetic field undergoes periodic reversals due to dynamo-action in the solar interior, through which the Sun’s magnetic field regenerates. The emergence of new magnetic field at the solar surface, after buoyantly rising through the convection zone, is clearly
visible due to the formation of dark spots (sunspots). However, current models of the solar dynamo are unable to self-consistently capture the formation of sunspots, due to the range of pressure scale heights needed to include the photosphere. Thus, dynamo models remain disconnected from sunspot observations. The cyclic evolution of the Sun’s magnetic field also has a clear impact on the structure of the solar atmosphere and outflowing wind above. Similarly, linking the evolution of different scales, from the buffeting of convective motions in the photosphere to the dissipation of Alfven waves in the solar wind. Modern models of the Sun, therefore, require the combination of expertise from a range of interconnected subject areas. In this talk, I will highlight some of the recent work from the WholeSun ERC Synergy grant (https://wholesun.eu), which
brings together expertise from five different host institutions across Europe. These works range from assessing the observational signatures of toroidal flux generation, to modelling small-scale energy injection at the base of the solar wind, and finally, estimating the large-scale variation in coronal structure and rotation during the solar cycle. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Tuesday June 11 2024 | Mustapha MEFTAH (LATMOS)
Les NanoSatellites au service de l’observation de la Terre et de l’univers Summary [click here] Le thème de la conférence portera sur l’observation de la Terre, le climat, les satellites et le NewSpace. Le 24 janvier 2021, une fusée Falcon 9 était lancée avec succès par SpaceX depuis Cap Canaveral, en Floride. Sous la coiffe du lanceur, il y avait 143 satellites. C’est à ce jour le plus grand nombre de satellites jamais déployés en une seule mission. Uvsq-Sat, premier nanosatellite français dédié à l’observation de variables climatiques essentielles, faisait partie de cette épopée. Depuis plus de trois ans, Uvsq-Sat observe la Terre et le Soleil de manière assidue. A 550 km d’altitude, ce satellite mesure la quantité d'énergie solaire réfléchie vers l'espace et la quantité d'énergie thermique émise par la Terre vers l’espace.
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Tuesday May 17 2022 | Recent hiree seminar Benjamin MAGNELLI (DAp)
A decade of the main-sequence of star-forming galaxies: New insights and perspectives on massive galaxy formation Summary [click here] The observed tight correlation between the star formation rate (SFR) and the stellar mass of star-forming galaxies (SFGs) is now well constrained over the last 10 Gyr of look-back time. This so-called main sequence (MS), whose normalization declines from z~3 to 0, is commonly interpreted as evidence that SFGs are evolving primarily through a steady and long star-forming mode, likely sustained by the cold gas accretion along the cosmic web. Over the last decade, a plethora of studies have investigated within this framework the physical properties of SFGs along and across the MS, establishing key scaling relations between, e.g., the stellar mass, gas content, and/or morphology of SFGs in the SFR-stellar mass plane. In this talk I will review past and recent observational evidences of this new MS paradigm and how it has shaped our understanding of the evolution of massive galaxies. Then, I will present the limitations of this simple paradigm, and in particular how it fails to explain the more diverse than anticipated population of MS galaxies (e.g., starburst hidden within the MS), the importance of secondary parameters (e.g., environment) and the transition of SFGs to quiescence. I will conclude by presenting future observational opportunities that can be used to investigate this hidden complexity within the main sequence and to further unveil the physics involved in the evolution of massive galaxies over cosmic time. |
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Tuesday November 15 2022 | Large project seminar Anaëlle MAURY (LFEMI)
Magnetic fields from star-forming cores to protostellar disks: a review of major contributions from the MagneticYSOs project Summary [click here] Wherever we have the means of observing them, magnetic fields are detected across the full spectrum of astrophysical environments, from our own Earth, to stars, and cosmological structures. Magnetic fields are also present at all scales and evolutionary stages of star-forming structures. They have long been suspected to play a key role in shaping the typical outcome of the star formation process, such as stellar mass, spin, and multiplicity, or even the fate of stars towards their ultimate stages.
In this talk, I will provide a global outlook on the progresses made in the recent years to characterize the role of magnetic fields during the embedded phases of the star formation process. Thanks to the development of observational capabilities and the parallel progress in numerical models capturing most of the important physics at work during star formation, the MagneticYSOs team successfully confronted detailed predictions of magnetized models to observational properties of the youngest protostars. I will present the physical processes and observational methods allowing to trace the magnetic field in embedded protostars, and review the main steps, success and limitations in comparing real observations to synthetic observations from the non-ideal MHD models. I will show how our work has shed light on the physical conditions required to ensure an efficient magnetic field coupling, and present unexpected results regarding the two main agents responsible for the coupling in star-forming cores: dust grains and ionized gas. Following this Ariane thread, I will argue our observational and theoretical findings support a novel scenario where the angular momentum problem for star formation may be actually “solved” not by the formation of large protoplanetary disks but by the combination of 1) lack of organized rotation motions at large envelope radii, 2) the inefficient angular momentum transport due to magnetic braking in the inner envelope (and angular momentum removed through rotating outflows generated by the presence of the magnetic field), and 3) a local origin of the angular momentum incorporated in the star–disk system. Reference review |
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Tuesday June 13 2023 | Postdoc seminar Ugo LEBREUILLY (LMPA)
Gas and dust evolution during the formation of protoplanetary disks Summary [click here] Protoplanetary disks are the consequence of angular momentum conservation during the protostellar collapse. Their formation is a complex process which includes numerous physical effects (non-ideal MHD, stellar feedback, gas and dust interactions, turbulence…). In this seminar, I will
present our recent works to better understand the formation of these disks. In the first part of the talk, I will focus on their gas content. I will show how modelling simultaneously the large scales of star forming regions and the small scales of protoplanetary disks allows us to constrain the statistical properties (mass, radius, temperature…) of these disks. The second part of the talk will be dedicated to the study of dust evolution. In particular, I will stress the consequences of this process not only for the formation of disks, but also for the formation of planets. |
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Wednesday December 6 2023 | Ute LISENFELD (University of Granada)
Molecular gas in galaxies in compact groups Summary [click here] Galaxies in compact groups live in an environment where constant interactions take place, both through tidal and through collisional processes. They are are therefore ideal objects to study the influence of the environment on galaxy properties and evolution, in particular the quenching of star formation.
The atomic hydrogen is extremely affected by this environment, with high deficiencies in many cases, whereas the molecular gas content is on average relatively normal in most galaxies. However, the properties of the molecular gas are drastically different if one selects those galaxies, based on their mid-infrared colours from the Spitzer and WISE satallites, that are in transitioning phase from active to quiescent which have have a considerably lower molecular-to-stellar mass and star formation efficiency. In my talk I will present result form observations of the molecular gas and star formation in galaxies in compact groups, both from single dish and interferometric observations, that show indications that in many cases the molecular is highly perturbed (and therefore unable to form stars) in compact groups. |
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Tuesday February 6 2024 | Stefanie WALCH-GASSNER (University of Köln)
The impact of stellar feedback on different scales and galactic environments Summary [click here] Stellar feedback refers to the processes by which massive stars release energy, radiation and material into their surroundings, influencing the structure and evolution of the galaxies in which they reside. Understanding the impact of stellar feedback on different galactic environments is crucial for developing a comprehensive picture of galaxy formation and evolution. In this context, different galactic environments refer to regions within a galaxy that differ in their physical conditions, such as the average gas density, temperature, or metallicity.
We study the respective impact of stellar winds, ionizing radiation, and supernovae in modern simulations of the multi-phase interstellar medium in parts of galaxies within the SILCC project, which I will present in this talk. From these galactic scale simulations we find that ionizing radiation is the most important factor in regulating the star formation rate, while supernova over-pressure the gas substantially, thus driving a galactic outflow. |
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Tuesday February 13 2024 | Jonathan TAN (Chalmers & University of Virginia)
A Light in the Dark - Massive Star Birth Through Cosmic Time Summary [click here] Massive stars are important thoughout the universe, but their formation remains poorly understood. I review current understanding of how massive stars and star clusters form in our Galaxy, including models for how star formation is triggered in giant molecular clouds and tests of how individual massive stars form from smaller scale clumps and cores. Finally, I discuss how massive star formation may have been different in the very early universe and how the first stars may have seeded the supermassive black holes powering active galactic nuclei. |
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Tuesday May 7 2024 | Thomas HENNING (Max Planck Institute for Astronomy, Heidelberg)
Towards New Horizons: Star and Planet Formation with JWST Summary [click here] The James Webb Space Telescope (JWST) is the most powerful space-based infrared observatory ever built. The first part of the talk will discuss the main structures of the telescope and its assembly after launch. The talk will then highlight the extremely exciting science results in the field of star and planet formation, including the nature of protostars and planet-forming disks. A special emphasis will be placed on the accretion process and the chemical building blocks of disks and what we can learn from infrared spectroscopy regarding the inventory of material in the terrestrial planet-forming zone. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday September 10 2024 | Postdoc seminar Maxime LOMBART (LFEMI)
How to treat dust coagulation/fragmentation in 3D hydrodynamic simulations ? Summary [click here] Particles coagulation and fragmentation are ubiquitous (raindrop formation, air pollution, combustion, polymerization, astrophysics) and mathematically described by the Smoluchowski coagulation and the fragmentation equations. Several processes such as gas/particles dynamics, chemical reaction and radiative transfer depends on the evolution of the particle size distribution governed by coagulation/fragmentation. Tracking the size evolution of particles in 3D simulation is key for understanding, for instance, cloud formation and planet formation. Therefore, these equations must be accurately solved while preserving computational costs, which is a tremendous numerical challenge. However, current algorithms for solving coagulation/fragmentation suffer from an over-diffusion in the conditions of 3D simulations. To tackle this challenge, we developed a discontinuous Galerkin scheme to efficiently solve the conservative form of the coagulation/fragmentation equation. In particular, we aim to perform the first 3D simulations of dusty protoplanetary discs and protostellar collapse that include realistic coagulation/fragmentation. |
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Tuesday November 19 2024 | Postdoc seminar Jose Ignacio "Nacho" ANEZ LOPEZ (LFEMI)
Signatures of magnetic braking in Class 0 protostars ? Exploring the gas kinematics in magnetized models of low-mass star formation Summary [click here] The role of magnetic braking in regulating gravitational collapse and circumstellar disk during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disk sizes and the launching of high-velocity collimated jets, we aim at more direct tests of the magnetic braking in observations. The study of polarized dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process.
In the present work, we have used both non-ideal MHD models and synthetic observations from the radiative transfer of protostellar formation to put constraints on the magnetically-regulated disk formation scenario. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetized model shows a higher angular momentum redistribution above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O(2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetized model similar to the characteristics found in observational work. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from radial motions. |
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Thursday October 13 2022 | Group seminar Jordan PHILIDET (Max Planck Institute for Solar System Research, Göttingen)
Impact of turbulent convection on stellar oscillations Summary [click here] The study of the global oscillations of stars, which is at the heart of asteroseismology, has enabled us to make significant progress in our
understanding of stellar physics. In Sun-like stars, these oscillations are affected by the turbulent motions entailed by the convective instability in the envelope. In addition to modifying the resonant frequencies of the modes, turbulent convection is also responsible for their excitation, as well as part of their damping. As such, solar-like oscillations gives us access to crucial information about stellar convection, which constitutes one of the main obstacles towards better stellar modelling. However, in order to exploit the wealth of asteroseismic data at our disposal to better constrain the properties of stellar convection, it is necessary to theoretically model the relation between these properties and the asteroseismic observables (namely the amplitude of the modes, their lifetime, and the amount by which convection shifts their frequencies, referred to as surface effects).
In this talk, I will first focus on solar-like acoustic modes. Traditional approaches to study the effect of convection thereon are either based on parametric empirical formulations, or else on 3D simulations. These approaches show unavoidable limitations, among which the impossibility to realistically describe the full turbulent cascade, and especially the turbulent dissipation of kinetic energy. Here, I will present a new alternative theoretical framework designed to circumvent these limitations, based on Lagrangian stochastic models. I will demonstrate how, under reasonable assumptions, this sort of formalism can lead to simultaneous theoretical estimates for the amplitude, lifetime and surface effect of the acoustic modes, directly as a function of the turbulent fluctuations caused by convection, thus allowing to constrain turbulent convection models through the direct comparison of these estimates with observed mode properties. The second part of this talk will focus on another kind of oscillations, namely the inertial modes recently observed on the surface of the Sun. These modes propagate under the action of rotation, through the Coriolis acceleration, and, for the most part, are predicted to be stable, meaning that they are likely also excited by turbulent convection, just like acoustic modes. In order to test that hypothesis, I will present a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes, which are described by means of a 2D linear wave equation, under the β-plane approximation. Based on the general agreement between the predicted and observed inertial mode amplitudes, I will show that the (linearly stable) solar inertial modes are indeed excited by turbulent convection. This formalism also shows that the power in high azimuthal order spectra is not easily separable into individual modes, thus complicating the interpretation of the observations. |
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Friday November 25 2022 | Special seminar Leïla GODINAUD, Anne DECOURCHELLE, Fabio ACERO (DAp)
De Nova Stella : Séminaire d'anniversaire des 450 ans de la supernova de Tycho Summary [click here] En novembre 1572, l'observation de l'apparition d'une "nouvelle étoile" dans le ciel et les observations détaillées réalisées par Tycho Brahe ont transformé la façon dont nous voyons et comprenons notre Univers. Pour marquer l'événement, nous proposons un séminaire en trois parties allant de l'observation historique de l'événement jusqu'aux connaissances actuelles sur la supernova SN1572 et son vestige.
La conférence sera suivie d'un café gourmand dans le hall de la salle Galilée. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH De Nova Stella: 450th anniversary seminar for Tycho's supernova In November 1572, the observation of the apparition of a "new star" in the sky and the detailed observations made by Tycho Brahe transformed the way we see and understand our Universe. To mark the event we propose a three-part seminar ranging from the historical observation of the event to the current knowledge about the supernova SN1572 and its remnant. The conference will be followed by a gourmet coffee in the hall of the Galilee room. |
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Tuesday January 17 2023 | Ludovic PETITDEMANGE (LRA/LERMA)
Dynamo action and angular momentum transport in simulated stellar radiative zones Summary [click here] The evolution of a star is influenced by its internal rotation dynamics through transport and mixing mechanisms, which are poorly understood. Magnetic fields can play a role in transporting angular momentum and chemical elements, but the origin of magnetism in radiative stellar layers is unclear. Using global numerical simulations, we identify a subcritical transition to turbulence due to the generation of a magnetic dynamo. Our results have many of the properties of the theoretically-proposed Tayler-Spruit dynamo mechanism, which strongly enhances transport of angular momentum in radiative zones. It generates deep toroidal fields that are screened by the stellar outer layers. This mechanism could produce strong magnetic fields inside radiative stars, without an observable field on their surface. Magnetic fields generated by dynamo action appear as a process to trigger turbulence in stellar interiors. Depending on the parameters or initial conditions, we report different dynamo branches that could explain stellar magnetism and the rotation profiles observed for stars having a thick radiative envelope. |
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Tuesday April 30 2024 | Postdoc seminar Dinil PALAKKATHARAPPIL (LDE3)
Precise Stellar Age Determination of Stars with Gaia and TESS: Prospects and Insights for the upcoming PLATO and HAYDN Missions Summary [click here] Determining precise star ages is essential for insights into stellar, exoplanetary, and galactic evolution, but it remains a complex challenge. The two most precise techniques are (1) asteroseismology, which is the study of the internal structure of stars by means of intrinsic global oscillations, and (2) analysis of stellar clusters where stars are formed from the same molecular cloud at the same time, and the observed colour-magnitude
diagram is a strong diagnostic of the cluster age. In this talk, I will combine these two techniques to determine the age of intermediate age open cluster NGC 2477. We used astrometry and colour from Gaia to extract membership probabilities and colour information about the stars in clusters, while pulsating stars were found using TESS data. Observations are complemented with asteroseismic modelling using MESA and GYRE to obtain the precise age of the cluster. In the second part of the talk, I will detail my contribution to the development of the ground segment of the ESA M3 PLATO mission to obtain stellar mass, radius, and age as well as several rotation and magnetic activity indicators for which CEA is responsible. I will conclude by showcasing some applications of the PLATO pipeline using Kepler and TESS data. |
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Tuesday September 17 2024 | Postdoc seminar Arghyadeep PAUL (LDE3)
Star-Planet Magnetic Interactions: From Solar System to Exoplanets Summary [click here] The interaction between stellar winds and planetary magnetospheres has been a focus of research for decades. Within the heliospheric context, star-planet interactions similar to that between the Sun and the Earth reshapes the planet’s local magnetic environment leading to the formation of magnetospheres. Magnetic reconnection within these magnetospheres gives rise to helical magnetic flux ropes known as flux transfer events (FTEs). We developed a two-way coupled magnetosphere-ionosphere model to investigate the impact of FTEs on the planet’s ionosphere. For a specific case of an Earth like planet, the field aligned currents generated by these FTEs closely resemble observations of discrete dayside auroral arcs, suggesting FTEs to be a probable cause. In the context of exoplanetary systems, star-planet interactions vary depending on the planet's orbital location. For close-in orbits located in the sub-Alfvénic stellar wind, the Poynting flux generated by star-planet interactions can propagate toward the star, giving rise to stellar chromospheric hotspots. Our current work characterises and quantifies the efficiency of this energy transfer
between the planet and the star, revealing that a significant portion of the energy never reaches the star and is reflected by the stellar transition region back toward the planet. The firm detection and characterisation of such magnetic interactions from observations of chromospheric hotspots would also lead to constraints on the amplitude of the magnetic field of exoplanets, to which we are blind so far. Future research, utilising the previously developed magnetosphere-ionosphere model, will also explore how the presence or absence of a planetary ionosphere influences the Poynting flux generated by the planet. |
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Tuesday October 8 2024 | Lorenzo DUCCI (University of Tübingen)
Accretion anomalies: a journey through fast transients phenomena and accretion instabilities in X-ray binaries Summary [click here] Accretion onto compact objects, such as black holes and neutron stars, is a fundamental process in astrophysics, powering some of the most luminous objects in the universe. In X-ray binaries, the accretion of matter from a companion star onto the compact object leads to the emission of intense X-ray radiation. While the general framework of accretion is well understood, there exist peculiar and enigmatic fast flaring activities exhibited by different types of X-ray binaries, which challenge our current understanding of these systems.
In this seminar, I will delve into the diverse flaring behaviors observed in various X-ray binaries, highlighting their extreme properties. I will then discuss the possible mechanisms that have been proposed to explain these events. By exploring them, we can gain insights into the underlying physics of accretion and the behavior of matter in extreme environments. Finally, I will outline the future prospects for studying these enigmatic events, including the potential for new discoveries with upcoming X-ray missions. |
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Tuesday November 12 2024 | Joel ONG (Institute for Astronomy, University of Hawaii at Manoa)
New Views into Stellar Rotation from Asteroseismology Summary [click here] Asteroseismology — the analysis and interpretation of stellar oscillations — remains the only means by which we may directly inspect the properties of stellar interiors. I will describe new theoretical developments regarding the interpretation of asteroseismic rotational signatures in a particular class of variable stars — red-giant solar-like oscillators. While waves in most variable stars propagate in a single mode cavity, I formulate a description of multicavity oscillations in these red giants by way of a physical analogy where they behave as "acoustic molecular orbitals". I describe two recent observational results which this formalism has helped us to interpret: 1. the detection of engulfment signatures in evolved red giants, and 2. direct constraints on the misalignment angle between the rotational axis of the core and of the envelope, in the central star of a spin-orbit-misaligned planetary system. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday February 8 2022 | Special seminar Ingo WALDMANN (UCL)
Deep learning in exoplanet characterisation Summary [click here] The use of machine and deep learning is prevalent in many fields of science and industry and is now becoming more widespread in extrasolar planet and solar system sciences. Deep learning holds many potential advantages when it comes to modelling highly non-linear data, as well as speed improvements when compared to traditional analysis and modelling techniques. However, their often ‘black box’ nature and unintuitive decision processes, are a key hurdle to their broader adoption. In this seminar, I will give an overview of deep learning approaches used in exoplanet characterisation and discuss our recent work on developing Explainable AI (XAI) approaches. XAI is a rapidly developing field in machine learning and aims to make ‘black box’ models interpretable. By understanding how different neural net architectures learn to interpret atmospheric spectra, we can derive more robust prediction uncertainties as well as map information content as function of wavelength. As data and model complexities are bound to increase dramatically with the advent of JWST and ELT measurements, robust and interpretable deep learning models will become valuable tools in our data analysis repertoire. |
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Tuesday November 22 2022 | Recent hiree seminar Antonio GARCIA MUÑOZ (DAp)
Exoplanet atmospheres: An anticipated revolution Summary [click here] The launch of the JWST less than a year ago is expected to set a turning point in exoplanet science, which is progressively transitioning from detection and population statistics to in-detail characterization of the exoplanets’ atmospheres. In this talk, I will present an admittedly biased
perspective of what this may represent for our understanding of exoplanets, and how ongoing theoretical work and future telescopes may build upon JWST’s legacy. |
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Tuesday December 6 2022 | David ALONSO (Oxford Department of Physics)
Modeling the birth and growth of the cosmic web Summary [click here] In the last 10-20 years we have been able to observe vast swathes of the Universe at different wavelengths, allowing us to build high-sensitivity maps of different projected cosmic properties. The statistical correlation between these properties and the density inhomogeneities that underlie the cosmic large-scale structures can then be used to reconstruct the spatial distribution of fundamental cosmological and astrophysical quantities, as well as their evolution in time. In this talk, I will describe a number of methods used to carry out this kind of tomographic reconstruction, present measurements of fundamental properties (structure growth, gas pressure, star formation rate density) resulting from their application to existing data, and discuss the potential of near-future "Stage-IV" experiments to improve on and benefit from these methods, in their quest to improve our understanding of fundamental physics. |
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Thursday June 29 2023 | Julien DE WIT (Department of Earth, Atmospheric and Planetary Science, MIT, USA)
Robustly Characterizing Exoplanets in a New Era of High-Precision Transit Measurements Summary [click here] With a new-generation of great observatories coming online this decade, unprecedented insights into exoplanets will soon be within reach. Observatories such as the James Webb Space Telescope (JWST) notably enable the study of atmospheres around terrestrial exoplanets and can reveal tri-dimensional structures in the atmospheres of their larger counterparts. Robustly leveraging new observations to reach such achievements will however require extra care as the models currently used may not be up to par with their precision.
During this presentation, I will introduce work done by MIT’s Disruptive Planets group and collaborators towards supporting the robust in-depth characterization of exoplanets. I will specifically discuss how not accounting for the true shape of a planet can lead to a misinterpretation of its interior properties as well as atmospheric structure; how the current state of our understanding of light-matter interactions can similarly affect our interpretation of planetary spectra and thus inferences regarding their atmospheric properties; and how the current state of emission spectrum models for stars may even prevent from disentangling between the contribution of a planet and its host star, to start with. I will also present possible ways to address these challenges. I will end with a step-by-step roadmap to the robust characterization of temperate terrestrial planets with JWST, which includes habitability assessment. |
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Tuesday September 5 2023 | Postdoc seminar Lucie BAUMONT (LCS)
Galaxy Cluster Cosmology with fgas Summary [click here] The current Standard Model of Cosmology has successfully explained many phenomena, but it predicts that the majority of the Universe consists of dark matter and dark energy, whose properties are poorly understood. Because huge volumes collapse to form galaxy clusters, the largest known gravitationally bound structures, they are an ideal laboratory to study the Dark Universe. In fact, the ratio of baryonic matter to total matter in a massive cluster, fgas, can be considered representative of the matter content of the Universe as a whole. Measurements of fgas from the heaviest, dynamically relaxed galaxy clusters place powerful constraints on cosmological parameters as well as the dark energy equation of state. I will discuss constraints derived from fgas measurements using a multi-wavelength set of X-ray and optical data and provide outlook on the future of this measurement in the age of precision cosmology. |
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Tuesday October 3 2023 | Postdoc seminar Lara PANTONI (LFEMI)
Dust millimetre emission in nearby galaxies with NIKA2 (IRAM-30m): major challenges and latest results of the IMEGIN Large Program Summary [click here] The millimetre part of the spectrum is one of the least explored parts of a galaxy’s spectral energy distribution (SED), yet it contains emissions from three fundamentally important physical processes. These processes are thermal emission from dust, free-free emission from ionized gas and synchrotron emission from relativistic charged particles moving in the galactic magnetic field. The NIKA2 camera (IRAM-30m telescope), observing at 1.15 mm and 2 mm, provides additional data points for input into the comprehensive SED models and allows us to:
During my presentation, I will focus on the major challenges linked with data processing, uncertainty propagation, and large-scale emission filtering in NIKA2 maps (due to atmosphere removal during the data reduction process). I will show and discuss the latest significant results on NGC891 (Katsioli et al. 2023); NGC4254 (Pantoni et al. in prep.); NGC2976 and NGC2146 (Ejlali et al. in prep.); millimetre morphology (Nersesian et al. in prep.); future perspectives/applications. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Tuesday March 5 2024 | Recent hiree seminar Emmanuel BERTIN (LCEG)
Wide-field imaging meets deep learning: new challenges, new opportunities Summary [click here] Since the 19th century, wide-field imaging has significantly advanced numerous fields of Astrophysics, spanning from the study of solar system bodies to observational cosmology. Ongoing and future optical/near-infrared imaging surveys face many interesting data analysis challenges, especially in time domain astronomy, in a context where detector technology enable wide-field observations at increasingly high frame rates. Through concrete examples, I will show with how Deep Learning techniques offer promising solutions to address such challenges, and provide new scientific opportunities. |
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Tuesday April 23 2024 | Florent MERTENS (Observatoire de Paris)
Unveiling the Physics of the Cosmic Dawn and the Epoch of Reionisation in the SKA era Summary [click here] Direct observation of the Cosmic Dawn and Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are underway aiming at statistically detecting the 21-cm signal fluctuations from the EoR and CD, and paving the way for the SKA EoR CD experiment which will be capable of directly image the large-scale neutral hydrogen structures from these distance epochs. In this talk, I will present recent results from the LOFAR-EoR and NenuFAR Cosmic Dawn collaborations. After publishing the deepest upper limit on the signal power-spectra at z~9, which has made it possible to set constraints on the physics of the IGM during the EoR, the LOFAR-EoR team is progressing towards a deeper upper limit on a broader range of redshift. On the Cosmic Dawn front, the NenuFAR CD team recenty published a first upper limit on the signal power-spectra at z~20. These new results will be discussed, as well as the improvements in instrument calibration and foreground mitigation that enabled these achievements. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Wednesday October 16 2024 | Daisuke NAGAI (Yale University, USA)
Cosmology in the Era of Multi-Wavelength Astronomical Surveys Summary [click here] We are entering the golden age of multi-wavelength astronomical surveys. In the 2020s, a plethora of surveys (such as Euclid, eROSITA, Rubin-LSST, Simons Observatory, and CMB-S4) are underway or planned to provide unprecedented insights into cosmology and galaxy formation. In this talk, I will provide a brief overview of significant scientific opportunities and the notable challenges in the era of big data, with highlights on recent advances in computational modeling and the integral roles played by artificial intelligence and machine learning. |
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Tuesday October 11 2022 | Françoise COMBES (LERMA)
Overview of Cosmology with SKA, the Square Kilometer Array Summary [click here] SKA is a new technology radio-telescope array, about two orders of magnitude more sensitive and rapid in sky surveys than present instruments. It will be able to detect and measure the redshifts of billions of galaxies at the redshifts up to z=2, to probe through baryonic acoustic oscillations the nature of dark energy; it will probe the cosmic dawn of the universe, just afer recombination, and during the epoch of reionisation (z=6-15); it will be the unique instrument to map the atomic gas in high redshift galaxies, and determine the amount and distribution of dark matter in the early universe. With SKA-VLBI, it will unveil the accretion and feedback processes near super-massive black holes, and results from precursors will be shown. We will discuss these exciting perspectives, which will concretize at the end of the decade. |
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Tuesday November 15 2022 | Large project seminar Anaëlle MAURY (LFEMI)
Magnetic fields from star-forming cores to protostellar disks: a review of major contributions from the MagneticYSOs project Summary [click here] Wherever we have the means of observing them, magnetic fields are detected across the full spectrum of astrophysical environments, from our own Earth, to stars, and cosmological structures. Magnetic fields are also present at all scales and evolutionary stages of star-forming structures. They have long been suspected to play a key role in shaping the typical outcome of the star formation process, such as stellar mass, spin, and multiplicity, or even the fate of stars towards their ultimate stages.
In this talk, I will provide a global outlook on the progresses made in the recent years to characterize the role of magnetic fields during the embedded phases of the star formation process. Thanks to the development of observational capabilities and the parallel progress in numerical models capturing most of the important physics at work during star formation, the MagneticYSOs team successfully confronted detailed predictions of magnetized models to observational properties of the youngest protostars. I will present the physical processes and observational methods allowing to trace the magnetic field in embedded protostars, and review the main steps, success and limitations in comparing real observations to synthetic observations from the non-ideal MHD models. I will show how our work has shed light on the physical conditions required to ensure an efficient magnetic field coupling, and present unexpected results regarding the two main agents responsible for the coupling in star-forming cores: dust grains and ionized gas. Following this Ariane thread, I will argue our observational and theoretical findings support a novel scenario where the angular momentum problem for star formation may be actually “solved” not by the formation of large protoplanetary disks but by the combination of 1) lack of organized rotation motions at large envelope radii, 2) the inefficient angular momentum transport due to magnetic braking in the inner envelope (and angular momentum removed through rotating outflows generated by the presence of the magnetic field), and 3) a local origin of the angular momentum incorporated in the star–disk system. Reference review |
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Tuesday December 13 2022 | Recent hiree seminar Réza ANSARI (DAp)
Cosmology with 21cm Intensity Mapping Summary [click here] Intensity Mapping (IM) has been proposed about 15 years ago as an efficient technique to perform cosmological surveys. The 21cm hyperfine transition of neutral hydrogen can indeed be used to map the 3D distribution of matter in the universe, over a wide range of redshifts, from z=0 to z=3 or even z=6, bringing complementary information to the optical surveys.
Since then, few dedicated instruments have been built (CHIME, Tianlai, BINGO) to explore the feasibility of the method; Other more ambitious instruments, such as HIRAX, CHORD or BINGO will be commissioned in the coming years. Intensity mapping surveys are also envisaged for SKA, in addition to the classical HI source surveys. After presenting the principle of 21 intensity mapping, I will briefly discuss its cosmological promises, as well as some of the associated instrumental and scientific challenges. I will then present some of the results of ongoing observations, focusing on Tianlai, and on PAON4. Tianlai is an international project that operates two pathfinder instruments, a cylinder array and a parabolic array, built in Xinjiang, in western China. PAON4 is a small test interferometer, located in Nançay, used to explore some of the technical aspects of compact radio arrays, operating in transit mode. |
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Tuesday January 10 2023 | Stéphane AUNE, Jean-Charles CUILLANDRE et Pierre ASTIER (CEA/SEDI, CEA/DAp et LPNHE)
20 years of MegaCam scientific operations on the Canada-France-Hawaii Telescope Summary [click here] Over its history CFHT has helped build or hosted dozens of instruments, operating from ultraviolet to mid-infrared wavelengths, attuned to Maunakea’s renowned observing conditions. One instrument, though, eclipses them all – MegaCam. It is in fact hard to imagine CFHT’s success today without the decisions made two decades ago to build what was then the largest digital focal plane in the world. MegaCam took engineering techniques to closely mount CCDs into a single camera to a whole new level. With its 1 degree field of view, MegaCam was able to tile the sky with images, creating rich and precise datasets on enormous scales that could be mined by the entire scientific community for decades. To date MegaCam has led to more peer reviewed science publications than all other instruments used at CFHT combined. It has been used to tune the model of Dark Energy and map vast regions of Dark Matter, unveil faint wisps of stars flung from interacting galaxies, reveal hundreds of free floating planets in star forming regions, and is currently directly contributing to a Stage IV dark energy measurement by providing the key ground-based photometry for the ESA Euclid space mission to derive photometric redshifts of hundreds of millions of galaxies over the northern sky. All of this is part of a revolution in wide field high-resolution optical imaging that CFHT pioneered for many years. Three speakers will offer a complete overview from technical aspects to the most impactful scientific result over these past two decades:
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Tuesday April 18 2023 | Judith IRWIN (Queen's University, Canada)
CHANG-ES — Past and Future Summary [click here] CHANG-ES (Continuum Halos in Nearby Galaxies — an EVLA Survey) is a project to observe 35 nearby galaxies that are edge-on to the line of sight to focus on their radio halos and the disk-halo connection. Wide-band VLA observations at L-band (1.5 GHz) and C-band (6.0 GHz) have provided opportunities to study in-band spectral indices, and observations in all four Stokes parameters with Rotation Measure Synthesis has led to a new understanding of the structure of kpc-scale magnetic fields in disk galaxies. This talk will highlight some of the results of the project and look to the future, as newly completed S-band (3.0 GHz) observations have filled in the L-band to C-band gap and led to the widest contiguous frequency coverage yet seen for galaxies. |
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Tuesday September 19 2023 | Arjen VAN DER WEL (Ghent University, Belgium)
Galaxy Evolution from z=3 to the Present Day: Recent Insights from JWST and VLT surveys Summary [click here] Thanks to an abundance of excellent observational data the evolution of the population of galaxies in terms of its stellar mass function, the distribution of star-formation across galaxies and their structural properties is known with good precision. The challenge we face, though, is to decipher the evolutionary paths of individual galaxies. Two roads to achieving this goal are 1) to reconstruct star-formation histories of individual galaxies through detailed modeling of high-quality spectra, and 2) attempt to define 'rules' for galaxy evolution that tell us how the collection of individual galaxies must evolve so that the population properties are reproduced. I will describe our recent progress along these lines, enabled by high-quality data from VLT spectroscopy of z~1 galaxies (the LEGA-C survey) and high-quality imaging data from HST (CANDELS) and JWST (CEERS). |
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Wednesday October 4 2023 | Joint DAp-DPhP seminar Roland BACON (CRAL)
WST - The Wide Field Spectroscopic Telescope Summary [click here] The WST project aim to study and built an innovative 10-m class wide-field spectroscopic survey telescope (WST) in the southern hemisphere with simultaneous operation of a large field-of-view (5 sq. degree) and high multiplex (20,000) multi-object spectrograph facility with both medium and high resolution modes (MOS), and a giant panoramic integral field spectrograph (IFS). The ambitious WST top-level requirements place it far ahead of existing and planned facilities. In just its first 5 years of operation, the MOS will target 250 million galaxies and 25 million stars at medium resolution + 2 million stars at high resolution, and 4 billion spectra with the IFS. WST will achieve transformative results in most areas of astrophysics. The combination of MOS and IFS spectroscopic surveys is one of the key aspects of the project. It is very attractive because of the high complementarity between the two approaches. I will detail this innovative point using the example of the MOS and MUSE surveys performed in the CDFS region. The project aims to be the next major post-ELT project. It is supported by a large consortium of very experienced institutes plus ESO, representing 9 European countries and Australia. |
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Tuesday November 28 2023 | Elisa CHISARI (University of Oxford)
A rising tide: galaxy alignments in the Stage IV era Summary [click here] A new generation of galaxy surveys is at our doorstep. By mapping billions of galaxies over the sky, they will deliver precision constraints on the cosmological model. The significant gains in constraining power are accompanied by the need to develop more accurate models of galaxy shapes. Several physical effects will come into play that could be neglected or simplified in previous analyses. I will discuss the constraining power of upcoming surveys, focusing mostly on Rubin Observatory, and the modelling challenges they face. Amongst such challenges, I will focus mostly on the “intrinsic alignments” of galaxies: alignments between their shapes that are likely to arise from tidal interactions between them. I will describe some new modelling strategies that could help reduce their impact on cosmological constraints. Finally, I will present some opportunities that arise for using intrinsic alignments themselves as a probe of cosmology and galaxy evolution. |
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Tuesday April 23 2024 | Florent MERTENS (Observatoire de Paris)
Unveiling the Physics of the Cosmic Dawn and the Epoch of Reionisation in the SKA era Summary [click here] Direct observation of the Cosmic Dawn and Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are underway aiming at statistically detecting the 21-cm signal fluctuations from the EoR and CD, and paving the way for the SKA EoR CD experiment which will be capable of directly image the large-scale neutral hydrogen structures from these distance epochs. In this talk, I will present recent results from the LOFAR-EoR and NenuFAR Cosmic Dawn collaborations. After publishing the deepest upper limit on the signal power-spectra at z~9, which has made it possible to set constraints on the physics of the IGM during the EoR, the LOFAR-EoR team is progressing towards a deeper upper limit on a broader range of redshift. On the Cosmic Dawn front, the NenuFAR CD team recenty published a first upper limit on the signal power-spectra at z~20. These new results will be discussed, as well as the improvements in instrument calibration and foreground mitigation that enabled these achievements. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Wednesday October 16 2024 | Daisuke NAGAI (Yale University, USA)
Cosmology in the Era of Multi-Wavelength Astronomical Surveys Summary [click here] We are entering the golden age of multi-wavelength astronomical surveys. In the 2020s, a plethora of surveys (such as Euclid, eROSITA, Rubin-LSST, Simons Observatory, and CMB-S4) are underway or planned to provide unprecedented insights into cosmology and galaxy formation. In this talk, I will provide a brief overview of significant scientific opportunities and the notable challenges in the era of big data, with highlights on recent advances in computational modeling and the integral roles played by artificial intelligence and machine learning. |
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Tuesday November 19 2024 | Postdoc seminar Jose Ignacio "Nacho" ANEZ LOPEZ (LFEMI)
Signatures of magnetic braking in Class 0 protostars ? Exploring the gas kinematics in magnetized models of low-mass star formation Summary [click here] The role of magnetic braking in regulating gravitational collapse and circumstellar disk during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disk sizes and the launching of high-velocity collimated jets, we aim at more direct tests of the magnetic braking in observations. The study of polarized dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process.
In the present work, we have used both non-ideal MHD models and synthetic observations from the radiative transfer of protostellar formation to put constraints on the magnetically-regulated disk formation scenario. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetized model shows a higher angular momentum redistribution above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O(2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetized model similar to the characteristics found in observational work. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from radial motions. |
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Tuesday April 5 2022 | Barbara OLMI (INAF, Italy)
Modeling Pulsar Wind Nebulae through their evolutionary phases Summary [click here] Pulsar wind nebulae are fascinating systems, powered by the central rotating compact star, emanating a wind in the form of a relativistic, magnetized, and cold plasma that fills the nebula. They are visible as bright non-thermal sources in a very broad range of energies, from radio to gamma-rays. Observed morphologies vary with the evolutionary phase, with middle-aged and old systems strongly affected by the interaction with the ambient medium. Modeling of these sources requires some carefulness when going through the various phases, with a comprehensive description still lacking.
Pulsar wind nebulae had been for a long time thought to contribute substantially to the positron excess in the CR spectrum at Earth -- potentially being the primary sources. In the last years, numerous evidence for efficient particle leakage by aged nebulae had been collected, showing up as quasi-monochromatic misaligned jets at X-rays in some cases, or in the form of extended TeV halos in others, reanimating somehow the interest in this class of objects. Here I will review our present knowledge of pulsar wind nebulae models through their different ages. |
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Tuesday April 19 2022 | Sacha BRUN et Olivier LIMOUSIN (DAp)
Solar Orbiter: the heliospheric explorer Summary [click here] Solar Orbiter - ESA M1 Mission - has entered its scientific phase at the end of November 2021, after a cruise phase of more than 18 months (and 2.2 Billion km) and the commissioning of the 10 instruments on board. CEA/IRFU played a key role in this mission, by providing the focal plane detector array of the X-ray telescope, STIX, based on in-house Caliste technology. In the meantime, solar physicists have been busy preparing the pipelines needed to handle the data sent by the instrument, and developing high performance numerical simulations of the Sun.
In this 2-voice seminar, we will relate the first 26 months of activities of the mission, covering both STIX calibration, its first light, the first solar flare detected, as well as the development of associated numerical simulations of the Sun and optimal scientific processing of instrumental data sent as the spacecraft gets closer and closer to the Sun (last perihelion was on 26 March 2022 at 0.32 AU), while in the meantime our star is increasing in intensity and in activity, with solar magnetic cycle 25 now well on its way. |
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Tuesday May 10 2022 | Group seminar Lev TITARCHUK (University of Ferrara, Italy)
Comptonization Problem and Its solution in Application to the Spectra of the Neutron Star and Black Hole Sources Summary [click here] In 2017 the work on the Comptonization (Sunyaev-Titarchuk) seen in the X-ray spectra of astrophysical sources was a candidate for the Nobel Prize in Physics. In this talk I provide all the details of the exciting prehistory of this topic and precise details of this discovery. The solution of this problem and its subsequent development and application to the spectra of accreting neutron star (NS) and black hole (BH) binaries reveals a lot of information on these objects. In particular, now we can unambiguously distinguish between a NS and a BH (Galactic or extragalactic) using correlations of their spectral indices vs mass accretion rate (or QPO frequency). I further demonstrate how we can determine a BH mass using this correlation. |
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Tuesday June 14 2022 | Diego GÖTZ et Aline MEURIS (DAp)
The MXT telescope on board SVOM: a new tool for time-domain and multi-messenger astrophysics Summary [click here] The Sino-French SVOM (Space based astronomical Variable Object Monitor) mission is ready for launch in 2023. This mission is dedicated to the study of Gamma-Ray Bursts and other transient and variable sources of the high-energy sky. On board SVOM there will be four instruments, ECLAIRs and GRM, with large field of views, operating in the hard X- and gamma-ray domain, and two narrow field instruments: the Visible Telescope (VT) and the Microchannel X-ray Telescope (MXT). The MXT is a novel kind of compact and light instrument based of the « Lobster Eye » optical concept, coupled to a low noise state-of-the art X-ray camera, the latter being designed and manufactured at CEA Irfu. For a total mass of 42 kg and a total power of 60 W, this instrument is composed of an optics system, a telescope tube in carbon fiber, a radiator, a camera and a data processing unit. The 9 kg camera consists of a focal plane assembly with a detector assembly and thermoelectrical coolers, a front-end electronics assembly, a calibration wheel assembly and a support structure assembly. We will first review the SVOM scientific objectifs and how MXT will contribute to reach them. Then we will present the MXT design in more detail, focussing on the CEA contribution, and finally we will present the results of the calibration campaign performed in 2021 before the delivery of the telescope to Cnes. |
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Tuesday September 6 2022 | Group seminar Nicolas SCEPI (CU Boulder)
Formation, Evolution and Radiation of Magnetized Accretion Disks around Black Holes Summary [click here] The impact of magnetic fields on the evolution and on the observational signatures of accretion disks is very uncertain. This uncertainty is mainly due to a lack of observational constraints on the magnetic field geometry or strength in accretion disks. However, even from a theoretical point of view our understanding of magnetized disks remains relatively poor. Indeed, analytic models of magnetized disks often need inputs from numerical simulations and numerical simulations of magnetized disks are difficult to perform and/or interpret. Because of this lack of magnetized disk models, standard disk models often reduce the magnetic field to a source of turbulence; turbulence through which the accretion can happen. While this simplification may hold for weakly magnetized disks, a large number of numerical simulations have shown that the role of a strong magnetic field goes far beyond producing turbulence. In particular, a strong magnetic field can produce powerful outflows, induce accretion through vertically elevated layers or non-axisymmetric structures, modify the time scales of accretion, enhance dissipation of gravitational energy in the disk and accelerate particles to very high energies. All of these effects dramatically affect the evolution and observational signature of accretion disks and open up new and exciting avenues to resolve outstanding problems of the standard accretion disk theory. In this talk, I will present an overview of my recent results on how strongly magnetized disks form, evolve and radiate. I will show in particular how strongly magnetized disks could explain events of very strong variability in AGNs, the flaring behavior of the Galactic center and the hardest emission in X-ray binaries. |
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Tuesday November 8 2022 | Philippe Laurent, Aymeric Sauvageon, Jérôme Rodriguez (DAp)
INTEGRAL : 20 ans dans l'espace pour une aventure de plus de 35 ans Summary [click here] Le laboratoire INTErnational d'Astrophysique des Rayons Gamma (INTEGRAL) a été lancé le 17 octobre 2002 de Baikonour (Kasakstan). Depuis lors il est resté sur son orbite elliptique de haute excentricité (environ 3 jours) effectuant 2568 révolutions (au 8 novembre 2022) autour de la Terre pour environ 530 Ms (méga secondes) d'observations scientifiques. Cette mission de taille moyenne transporte deux instruments principaux opérant dans le domaine spectral des rayons X durs et des rayons gamma mous (20keV-10 MeV), dont l'optique est basée sur le concept de masques codés : l'un est dédié à l'analyse spectrale fine avec des capacités d'imagerie modérées (SPI), l'autre est dédié à l'imagerie fine (ou presque) avec des capacités spectrales modérées (IBIS).
Ces 20 ans dans l'espace sont le résultat d'au moins 15 ans de développement du projet avant qu'INTEGRAL puisse dévoiler le mystère du ciel à haute énergie. Le Dap a été profondément impliqué dans cette aventure de longue haleine, depuis le tout début du développement du concept de la mission (grande implication dans les deux instruments, ISGRI et SPI, participation au centre de données, développement de l'analyse s/w, suivi de la caméra) jusqu'à l'analyse actuelle des données en temps réel et des archives, avec un grand nombre d'articles publiés dans tous les domaines permis par les capacités instrumentales et même au-delà. Nous allons, dans ce séminaire, résumer ces plus de 35 ans en présentant divers aspects du projet/de la mission, en nous concentrant particulièrement sur ceux où l'implication du Dap a été cruciale. Nous commencerons par une vue d'ensemble de l'histoire de la mission, des principales caractéristiques instrumentales et des objectifs scientifiques au lancement, puis nous nous concentrerons sur la caméra ISGRI, la couche de détection 20-250 keV du télescope IBIS, qui est suivie au Dap tous les jours. ISGRI a été développée à l'IRFU et a obtenu la plupart des résultats d'INTEGRAL. Nous conclurons par un aperçu rapide et évidemment biaisé de quelques résultats scientifiques obtenus au cours de ces 20 années. Le séminaire sera présenté en Français avec des diapositives en Anglais par Philippe Laurent, Aymeric Sauvageon et Jérôme Rodriguez, bien humblement au nom d'un grand nombre de collègues anciens et actuels. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH VERSION INTEGRAL : 20 years in space and for a 35+ years adventure The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) has been launch on October 17th, 2002 from Baikonour (Kasakstan). Since then it has remained on its high-eccentricity elliptical orbit (about 3 days) performing 2568 revolutions (as of Nov. 8th, 2022) around the Earth for around 530 megaseconds of scientific observations. This medium sized mission caries two main instruments operating in the spectral domain of hard X-rays/Soft Gamma-rays (20keV-10 MeV), whose optics is based on the concept of coded masks: one is dedicated to fine spectral analysis with moderate imaging capabilities (SPI) the other is dedicated to fine(-ish) imaging with moderate spectral capabilities (IBIS). These 20 years in space are the results of at least 15 years of project development before INTEGRAL could unveil the mystery of the high-energy sky. The DAp has been deeply involved in this long-term adventure from the very beginning of the mission concept development (large involvement in both instruments, ISGRI and SPI, participation to the data centre, development of the s/w analysis, monitoring of the camera) to the current analysis of real time and archival data, with a large number of published papers in all fields allowed by the instrumental capabilities and even beyond. We will, in this seminary, summarised these 35 years+ by presenting various aspects of the project/mission, focusing especially on those where the involvement of the Dap has been crucial. This will start with an overall overview of the mission history, the main instrumental characteristics and scientific goals at launch, followed by a focus on the ISGRI camera, the 20-250 keV detector layer of the IBIS telescope, which is followed-up at DAp every days. ISGRI has been developed at IRFU and obtained most of the INTEGRAL results. We will conclude with a quick and obviously biased overview of some scientific results obtained over these 20 years. The talks will be given by Philippe Laurent, Aymeric Sauvageon, and Jérôme Rodriguez humbly on behalf of many, many former and current colleagues and presented in French with slides in English. |
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Friday November 18 2022 | Group seminar Camille DIEZ (University of Tübingen)
Using Vela X-1 to understand accretion and wind structure in neutron star High-Mass X-ray Binaries (HMXBs) Summary [click here] The spectral and timing behaviour of HMXBs offers a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. The bright and persistent neutron star HMXB Vela X-1 is one of the key systems for such studies with both current and future instruments. It has a complex clumpy stellar wind, prominent cyclotron resonant scattering features (CRSFs) and strong flares. Understanding the variability of the systems on both short time scales of a few hundreds seconds and along its 9d orbit with current instruments enables us to make predictions for future observations with XRISM and Athena and to devise the best observational strategy for Vela X-1. Here, we analyse two new observations taken with NuSTAR and XMM-Newton at orbital phases ~0.5 and ~0.75 and follow the evolution of spectral parameters down to the pulse period (~300s) time-scale. The flux-dependency we observe in the spectral shape implies a change in the properties of the Comptonising plasma and the observed drop of the CRSF energy following a strong flare may indicate a change in the accretion geometry. The strong variability of absorption is due to the presence of a large-scale wind structure, such as accretion- and photoionisation wakes, combined with the variable line of sight as the neutron star moves along the orbit. In particular, we, for the first time, are able to trace the onset of the wakes with high time resolution and compare to predictions from simulations. |
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Tuesday November 29 2022 | Elisa COSTANTINI (SRON, Netherlands)
The cold and dusty medium in our Galaxy as seen in the X-rays Summary [click here] X-rays have unique advantages in studying absorption and scattering from interstellar dust. For example, sharp and deep absorption features of Mg, Si, O and Fe, which are the building blocks of silicates, fall in the X-ray band. Present X-ray observatories already delivered to us interesting results, challenging the common paradigm on interstellar dust chemical and physical characteristics. Future instruments will open up an unexplored window, revealing the most dense environments of our Galaxy.
In this talk I will illustrate the state-of-art of our understanding of dust as seen in the X-rays, and future prospects, using for example, the upcoming XRISM satellite. |
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Tuesday September 12 2023 | Alexander MOISEEV (NASA/GSFC and University of Maryland)
New Mission Concept: Compton Telescope with Coded Aperture Mask, and its Science Perspectives Summary [click here] The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel Explorer-class concept for a next-generation telescope covering the poorly explored hard X-ray and soft gamma-ray energies. This concept builds upon the heritage of past and current missions, improving sensitivity and, very importantly, angular resolution. GECCO uses the combined Coded Aperture Mask and Compton telescope techniques to employ the benefits of both: superior angular resolution provided by the deployable Coded Aperture Mask, and good background rejection and wide field-of-view (FoV) provided by the Compton telescope. It is being developed at NASA/GSFC in collaboration with other US and foreign institutions. GECCO observations will extend arcminute angular resolution to high-energy images of the Galactic plane, combining the
spectral capabilities of INTEGRAL/IBIS and the x-ray imaging of NuSTAR and eROSITA, and will make a bridge to the Fermi-LAT observations, enabling a broad potential for discoveries in the MeV γ-ray sky.
With the unprecedented angular resolution of the coded mask telescope combined with the sensitive, wide FoV Compton telescope, GECCO will focus on two main science objectives:
The GECCO design is based on the novel CZT Imaging calorimeter, which serves as a standalone Compton telescope and as a focal plane detector for the Coded Mask. It also is a powerful tool to measure the γ-radiation polarization. GECCO’s octagon-shaped active shield also serves as a powerful all-sky detector of gamma-ray bursts, prompting the instrument to slew towards the burst direction and localize it with the Coded Aperture’s arcminute accuracy. GECCO will operate in the 100 keV - 10 MeV energy range, with energy resolution of ~ 1% from 0.5 - 5 MeV. The Coded Aperture Mask provides the angular resolution of ∼1 arcmin with a 2 × 2 deg2 fully coded field-of-view, while the Compton telescope provides the angular resolution of 3◦ − 6◦ with a 60 × 60 deg2 field-of-view. The 3σ, 106 s sensitivity is expected to be about 10-5 MeV cm-2 s-1 over the entire energy range. Primary mode of observation is fixed pointing, with extended exposure of the regions of interest. However, as a standalone Compton telescope with wide FoV, Imaging Calorimeter will simultaneously provide wide-area sky exploration, significantly broadening GECCO’s observational scope. |
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Tuesday October 17 2023 | Postdoc seminar Sudip CHAKRABORTY (LEPCHE)
Unraveling the magnetic origin of dense non-equatorial ultrafast outflows in black hole X-ray binaries Summary [click here] Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Tuesday December 12 2023 | Stefano ANDREON (INAF-OA Brera, Milan, Italy)
First massive galaxy clusters emerging from the cosmic web at z~2 Summary [click here] In this talk I report upon our results on the intracluster medium (ICM) of two clusters at the time when first clusters start to emerge from the cosmic web, z~2. Results are derived from new, high resolution, deep SZ and X-ray data providing us with the measurement of the two most distant resolved pressure profiles. IDCSJ1426 cluster at z=1.75 has a core whose properties are not far from the final stage, while the remaining part of the cluster is experiencing a sizable gas, heat and entropy transfer. JKCS041 at z=1.80 is caught just after a major merger event as evidenced by its SZ-X-ray peak offset, its low central pressure, and its low Compton-Y parameter compared to its WL mass. Comparison with plausible descendents shows that its ICM will experience major changes at all radii. |
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Tuesday January 30 2024 | Florian PACAUD (University of Bonn)
Galaxy clusters and filaments in the first eROSITA all-Sky survey Summary [click here] Launched in July 2019, eROSITA was set to survey the sky in the soft X-ray band for four years and provide a much awaited update to the old ROSAT all-sky survey. In a few weeks, the collaboration will release the first all-sky map obtained from the first six month of observation. In this talk, I will present the instrument and its technical abilities. I will then explore the all-sky map and show selected results on individual local clusters and filaments. Finally, I will give a preview of the soon-to-be-released data products, including AGN and galaxy catalogs, and present some of the methods used for their cosmological analysis. |
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Tuesday March 19 2024 | Postdoc seminar Miguel FERNANDES MOITA (LISIS)
The Large Italian X-ray facility (LARIX) and the advancements in Laue lens development Summary [click here] The LARIX (LARge Italian X-ray facility) is a multi-project facility situated in the Scientific-Technological Pole of the University of Ferrara, housed in an underground building featuring a 100-meter-long tunnel with two large experimental rooms on each side. Dedicated to the development and testing of X- and Gamma-ray astronomy instrumentation, LARIX hosts two beamlines: the 12-meter-long LARIX-A in experimental room A, suitable for linearity tests of hard X-ray detectors, reflectivity measurements of X-ray reflector samples, and ground calibrations; and the 50-meter-long LARIX-T installed in the tunnel, ideal for testing gamma-ray reflectors and low-weight gamma-ray detector prototypes when requiring low-divergence beams or lengthy beamlines. This presentation will provide an overview of both installations, their instrumentation, past projects, and opportunities for access through collaborations or transnational access programs like AHEAD. Furthermore, we will discuss recent advancements in Laue lens development that we did in LARIX, particularly the TRILL project, supported by ASI, aimed at advancing the technological readiness of Laue lenses. Future goals include the ASTENA mission concept, submitted to the ESA program 'Voyage 2050', featuring a narrow field telescope (NFT) based on a focusing Laue lens with an energy passband from 50 to 700 keV and a 20-meter focal length, promising breakthroughs in sensitivity and angular resolution in this energy band. Additionally, we will explore innovative concepts such as a high-energy Laue lens spectro-polarimeter, inspired by the recent success of the IXPE mission. |
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Tuesday May 14 2024 | Recent hiree seminar Damien TURPIN (DAp)
The high-energy and multi-messenger transient sky in the eyes of the SVOM mission Summary [click here] Over the past decades, the study of highly energetic transient phenomena has led to several scientific breakthroughs from the discovery of r-process nucleosynthesis production sites to the first credible identification of ultra-high-energy cosmic-ray accelerators. In a general manner, these explosive events allow us to study extreme physical regimes (extreme gravity, extreme magnetic fields, relativistic shocks) and are unique probes to study the Universe properties up to high redshift. In the last 10 years, an unprecedented amount of new generation instruments observing the sky at all wavelengths and with all astrophysical messengers have significantly contributed to enlarging our vision of the transient sky phenomena. This observational revolution initiated by the gravitational wave interferometers has forced the astrophysical community to rapidly adapt to the new challenges of multi-messenger astronomy. In June 2024, the French community will have a leading role in this young research domain with a strong implication for the French lab. in the upcoming SVOM mission. In this seminar, I will review some of the latest results obtained by the scientific community exploring the high-energy transient sky, especially with a focus on the sources of interest for the SVOM mission. I will show how important will be the SVOM mission contribution to this exciting field of research that has probably revealed only a subset of its richness. |
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Tuesday June 25 2024 | Jérôme RODRIGUEZ, Bertrand CORDIER & Damien TURPIN (DAp)
The SVOM Launch: En route for the transient hunt Summary [click here] The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a Sino-French mission planned to be launched on June 22nd 2024 from Xichang China. The launch of the satellite opens the time when scientists will step up and start exploiting the data. SVOM, however, is already a long-term project during which the involvement of engineers, developers, and scientists has already been huge. In this presentation I will overview some of the aspects of the mission, the scientific programs and aims of the project, but also recall its history and try and acknowledge the involvement of many colleagues which permitted the achievement of this fantastic machine. I will also introduce some of the novelties and specificities of the mission, in particular all the aspects dedicated to the time domain astronomy and real time analysis, implying the development of a dedicated science center, specific tools, and shifts for a certain of us as burst advocates. Finally we will (try to) have a zoom with those of us that have attended the launch in China, so that they can share with us their feeling of the launch and first impressions post launch, in this opening scientific exploitation era.
The talk will be given in French with slides in English. |
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Tuesday October 8 2024 | Lorenzo DUCCI (University of Tübingen)
Accretion anomalies: a journey through fast transients phenomena and accretion instabilities in X-ray binaries Summary [click here] Accretion onto compact objects, such as black holes and neutron stars, is a fundamental process in astrophysics, powering some of the most luminous objects in the universe. In X-ray binaries, the accretion of matter from a companion star onto the compact object leads to the emission of intense X-ray radiation. While the general framework of accretion is well understood, there exist peculiar and enigmatic fast flaring activities exhibited by different types of X-ray binaries, which challenge our current understanding of these systems.
In this seminar, I will delve into the diverse flaring behaviors observed in various X-ray binaries, highlighting their extreme properties. I will then discuss the possible mechanisms that have been proposed to explain these events. By exploring them, we can gain insights into the underlying physics of accretion and the behavior of matter in extreme environments. Finally, I will outline the future prospects for studying these enigmatic events, including the potential for new discoveries with upcoming X-ray missions. |
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Tuesday November 5 2024 | Special seminar Jean-Marc BONNET-BIDAUD (DAp)
Chine, la civilisation du ciel transitoire Summary [click here] En Chine, dès l'époque classique des Han (IIe siècle AEC), le Ciel est conçu comme le miroir de la Terre, lieux d'évènements fortuits (éclipses, comètes, taches solaires, novae et supernovae,..) qui sont traqués et catalogués par une armée de scientifiques au service de l'empereur. De nombreux documents astronomiques ont ainsi été produits sur plusieurs millénaires, conservés ou récemment redécouverts par l’archéologie. C’est le cas notamment de la plus ancienne carte d’étoiles connue au Monde conçue il y a plus de 1200 et préservée dans un monastère bouddhique de la route de la Soie.
Avec la présentation de ces documents, on retracera les étapes, les instruments et les concepts qui ont mené l'astronomie chinoise ancienne à de nombreuses découvertes du ciel transitoire, à des époques où l’Europe n’envisageait encore le Ciel que comme une voute céleste fixe, éternelle et immuable. Avec ce recul de l’histoire, la mission franco-chinoise SVOM, qui va détecter et cataloguer les plus puissants phénomènes célestes variables, apparait aujourd'hui comme une continuation logique dans laquelle la France rejoint symboliquement la Chine dans la préoccupation du ciel transitoire. |
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Tuesday May 17 2022 | Recent hiree seminar Benjamin MAGNELLI (DAp)
A decade of the main-sequence of star-forming galaxies: New insights and perspectives on massive galaxy formation Summary [click here] The observed tight correlation between the star formation rate (SFR) and the stellar mass of star-forming galaxies (SFGs) is now well constrained over the last 10 Gyr of look-back time. This so-called main sequence (MS), whose normalization declines from z~3 to 0, is commonly interpreted as evidence that SFGs are evolving primarily through a steady and long star-forming mode, likely sustained by the cold gas accretion along the cosmic web. Over the last decade, a plethora of studies have investigated within this framework the physical properties of SFGs along and across the MS, establishing key scaling relations between, e.g., the stellar mass, gas content, and/or morphology of SFGs in the SFR-stellar mass plane. In this talk I will review past and recent observational evidences of this new MS paradigm and how it has shaped our understanding of the evolution of massive galaxies. Then, I will present the limitations of this simple paradigm, and in particular how it fails to explain the more diverse than anticipated population of MS galaxies (e.g., starburst hidden within the MS), the importance of secondary parameters (e.g., environment) and the transition of SFGs to quiescence. I will conclude by presenting future observational opportunities that can be used to investigate this hidden complexity within the main sequence and to further unveil the physics involved in the evolution of massive galaxies over cosmic time. |
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Tuesday November 22 2022 | Recent hiree seminar Antonio GARCIA MUÑOZ (DAp)
Exoplanet atmospheres: An anticipated revolution Summary [click here] The launch of the JWST less than a year ago is expected to set a turning point in exoplanet science, which is progressively transitioning from detection and population statistics to in-detail characterization of the exoplanets’ atmospheres. In this talk, I will present an admittedly biased
perspective of what this may represent for our understanding of exoplanets, and how ongoing theoretical work and future telescopes may build upon JWST’s legacy. |
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Tuesday June 6 2023 | Postdoc seminar Carlos GÓMEZ GUIJARRO (LCEG)
Understanding the role of compact star formation in galaxy evolution with ALMA and JWST Summary [click here] Massive elliptical galaxies in the local universe appear to have their high-redshift analogs in the form of extremely compact quiescent galaxies. Therefore, it seems that compact star formation appears to play a pivotal role in the evolutionary pathways of massive galaxies across cosmic history. However, it remains to be understood what this role is in the broader picture set by the main sequence and the scaling relations in galaxy evolution. From an ALMA survey at 1.1mm, we reveal that compact star formation appears to be the norm in massive star-forming galaxies, and sizes as extended as typical star-forming stellar disks are rare. A population of galaxies with modest star formation rates, but which exhibit extremely compact star formation with starburst-like depletion timescales unveils. Compact star formation appears as a physical driver of depletion timescales, gas fractions, and dust temperatures. Gas and star formation compression seems to be a mechanism that allows to hold their star formation rate even when their gas fractions are low and they are presumably on the way to quiescence. Another population of galaxies missed in the deep optical surveys but bright at far-IR/mm wavelengths unveils thanks to recent JWST observations. We present a study investigating the drivers of dust attenuation in massive galaxies in the JWST-era, showing how the stellar mass and morphology plays an important role, with evidence for more compact stellar profiles resulting in the obscuration of galaxies. |
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Thursday June 29 2023 | Julien DE WIT (Department of Earth, Atmospheric and Planetary Science, MIT, USA)
Robustly Characterizing Exoplanets in a New Era of High-Precision Transit Measurements Summary [click here] With a new-generation of great observatories coming online this decade, unprecedented insights into exoplanets will soon be within reach. Observatories such as the James Webb Space Telescope (JWST) notably enable the study of atmospheres around terrestrial exoplanets and can reveal tri-dimensional structures in the atmospheres of their larger counterparts. Robustly leveraging new observations to reach such achievements will however require extra care as the models currently used may not be up to par with their precision.
During this presentation, I will introduce work done by MIT’s Disruptive Planets group and collaborators towards supporting the robust in-depth characterization of exoplanets. I will specifically discuss how not accounting for the true shape of a planet can lead to a misinterpretation of its interior properties as well as atmospheric structure; how the current state of our understanding of light-matter interactions can similarly affect our interpretation of planetary spectra and thus inferences regarding their atmospheric properties; and how the current state of emission spectrum models for stars may even prevent from disentangling between the contribution of a planet and its host star, to start with. I will also present possible ways to address these challenges. I will end with a step-by-step roadmap to the robust characterization of temperate terrestrial planets with JWST, which includes habitability assessment. |
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Tuesday September 19 2023 | Arjen VAN DER WEL (Ghent University, Belgium)
Galaxy Evolution from z=3 to the Present Day: Recent Insights from JWST and VLT surveys Summary [click here] Thanks to an abundance of excellent observational data the evolution of the population of galaxies in terms of its stellar mass function, the distribution of star-formation across galaxies and their structural properties is known with good precision. The challenge we face, though, is to decipher the evolutionary paths of individual galaxies. Two roads to achieving this goal are 1) to reconstruct star-formation histories of individual galaxies through detailed modeling of high-quality spectra, and 2) attempt to define 'rules' for galaxy evolution that tell us how the collection of individual galaxies must evolve so that the population properties are reproduced. I will describe our recent progress along these lines, enabled by high-quality data from VLT spectroscopy of z~1 galaxies (the LEGA-C survey) and high-quality imaging data from HST (CANDELS) and JWST (CEERS). |
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Tuesday October 3 2023 | Postdoc seminar Lara PANTONI (LFEMI)
Dust millimetre emission in nearby galaxies with NIKA2 (IRAM-30m): major challenges and latest results of the IMEGIN Large Program Summary [click here] The millimetre part of the spectrum is one of the least explored parts of a galaxy’s spectral energy distribution (SED), yet it contains emissions from three fundamentally important physical processes. These processes are thermal emission from dust, free-free emission from ionized gas and synchrotron emission from relativistic charged particles moving in the galactic magnetic field. The NIKA2 camera (IRAM-30m telescope), observing at 1.15 mm and 2 mm, provides additional data points for input into the comprehensive SED models and allows us to:
During my presentation, I will focus on the major challenges linked with data processing, uncertainty propagation, and large-scale emission filtering in NIKA2 maps (due to atmosphere removal during the data reduction process). I will show and discuss the latest significant results on NGC891 (Katsioli et al. 2023); NGC4254 (Pantoni et al. in prep.); NGC2976 and NGC2146 (Ejlali et al. in prep.); millimetre morphology (Nersesian et al. in prep.); future perspectives/applications. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Wednesday December 6 2023 | Ute LISENFELD (University of Granada)
Molecular gas in galaxies in compact groups Summary [click here] Galaxies in compact groups live in an environment where constant interactions take place, both through tidal and through collisional processes. They are are therefore ideal objects to study the influence of the environment on galaxy properties and evolution, in particular the quenching of star formation.
The atomic hydrogen is extremely affected by this environment, with high deficiencies in many cases, whereas the molecular gas content is on average relatively normal in most galaxies. However, the properties of the molecular gas are drastically different if one selects those galaxies, based on their mid-infrared colours from the Spitzer and WISE satallites, that are in transitioning phase from active to quiescent which have have a considerably lower molecular-to-stellar mass and star formation efficiency. In my talk I will present result form observations of the molecular gas and star formation in galaxies in compact groups, both from single dish and interferometric observations, that show indications that in many cases the molecular is highly perturbed (and therefore unable to form stars) in compact groups. |
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Tuesday January 23 2024 | Francisca KEMPER (Institute of Space Sciences (ICE-CSIC) / ICREA / IEEC, Spain)
The interstellar dust reservoir in galaxies Summary [click here] The evolution of interstellar dust reservoirs, and the evolution of galaxies themselves go hand-in-hand, as the presence of dust alters evolutionary drivers, such as the interstellar radiation field and the star formation history, while at the same time, the dust is being formed and altered by processes taking place in galaxies. However, far-infrared and submillimeter studies have revealed enormous dust masses at high redshifts that are difficult to explain with dust production from evolved stars (the so-called "dust budget problem"), while in the nearby universe there is also a significant mismatch between the dust production rate and the dust mass observed in the interstellar medium of galaxies. I will go over some possible explanations in an attempt to find a way forward towards a solution to this seeming discrepancy. |
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Tuesday April 2 2024 | James DUNLOP (University of Edinburgh)
Charting the emergence of the first galaxies with JWST Summary [click here] The James Webb Space Telescope (JWST) is transforming our view of galaxy formation and evolution in the young Universe. I will provide an overview of the latest results from the PRIMER survey, the largest JWST Cycle-1 “Galaxies” programme which, in combination with other public JWST imaging, is now enabling us to chart the early growth of the galaxy population back to within ~300 million years of the Big Bang. Specifically, I will present and discuss the first robust determination of the evolving galaxy luminosity function extending out to redshifts z~13, as well as new measurements of the galaxy stellar mass function reaching out to z~9. I will then attempt to interpret these results in the context of our current understanding of the evolving dark matter halo mass function and the efficiency with which galaxies are able to convert their baryons into stars. Finally, I will discuss the prospects for future progress with JWST and with the Atacama Large Millimeter Array (ALMA), including the possibility of uncovering the emergence of the first galaxies. |
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Tuesday May 7 2024 | Thomas HENNING (Max Planck Institute for Astronomy, Heidelberg)
Towards New Horizons: Star and Planet Formation with JWST Summary [click here] The James Webb Space Telescope (JWST) is the most powerful space-based infrared observatory ever built. The first part of the talk will discuss the main structures of the telescope and its assembly after launch. The talk will then highlight the extremely exciting science results in the field of star and planet formation, including the nature of protostars and planet-forming disks. A special emphasis will be placed on the accretion process and the chemical building blocks of disks and what we can learn from infrared spectroscopy regarding the inventory of material in the terrestrial planet-forming zone. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday April 19 2022 | Sacha BRUN et Olivier LIMOUSIN (DAp)
Solar Orbiter: the heliospheric explorer Summary [click here] Solar Orbiter - ESA M1 Mission - has entered its scientific phase at the end of November 2021, after a cruise phase of more than 18 months (and 2.2 Billion km) and the commissioning of the 10 instruments on board. CEA/IRFU played a key role in this mission, by providing the focal plane detector array of the X-ray telescope, STIX, based on in-house Caliste technology. In the meantime, solar physicists have been busy preparing the pipelines needed to handle the data sent by the instrument, and developing high performance numerical simulations of the Sun.
In this 2-voice seminar, we will relate the first 26 months of activities of the mission, covering both STIX calibration, its first light, the first solar flare detected, as well as the development of associated numerical simulations of the Sun and optimal scientific processing of instrumental data sent as the spacecraft gets closer and closer to the Sun (last perihelion was on 26 March 2022 at 0.32 AU), while in the meantime our star is increasing in intensity and in activity, with solar magnetic cycle 25 now well on its way. |
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Tuesday June 14 2022 | Diego GÖTZ et Aline MEURIS (DAp)
The MXT telescope on board SVOM: a new tool for time-domain and multi-messenger astrophysics Summary [click here] The Sino-French SVOM (Space based astronomical Variable Object Monitor) mission is ready for launch in 2023. This mission is dedicated to the study of Gamma-Ray Bursts and other transient and variable sources of the high-energy sky. On board SVOM there will be four instruments, ECLAIRs and GRM, with large field of views, operating in the hard X- and gamma-ray domain, and two narrow field instruments: the Visible Telescope (VT) and the Microchannel X-ray Telescope (MXT). The MXT is a novel kind of compact and light instrument based of the « Lobster Eye » optical concept, coupled to a low noise state-of-the art X-ray camera, the latter being designed and manufactured at CEA Irfu. For a total mass of 42 kg and a total power of 60 W, this instrument is composed of an optics system, a telescope tube in carbon fiber, a radiator, a camera and a data processing unit. The 9 kg camera consists of a focal plane assembly with a detector assembly and thermoelectrical coolers, a front-end electronics assembly, a calibration wheel assembly and a support structure assembly. We will first review the SVOM scientific objectifs and how MXT will contribute to reach them. Then we will present the MXT design in more detail, focussing on the CEA contribution, and finally we will present the results of the calibration campaign performed in 2021 before the delivery of the telescope to Cnes. |
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Tuesday October 11 2022 | Françoise COMBES (LERMA)
Overview of Cosmology with SKA, the Square Kilometer Array Summary [click here] SKA is a new technology radio-telescope array, about two orders of magnitude more sensitive and rapid in sky surveys than present instruments. It will be able to detect and measure the redshifts of billions of galaxies at the redshifts up to z=2, to probe through baryonic acoustic oscillations the nature of dark energy; it will probe the cosmic dawn of the universe, just afer recombination, and during the epoch of reionisation (z=6-15); it will be the unique instrument to map the atomic gas in high redshift galaxies, and determine the amount and distribution of dark matter in the early universe. With SKA-VLBI, it will unveil the accretion and feedback processes near super-massive black holes, and results from precursors will be shown. We will discuss these exciting perspectives, which will concretize at the end of the decade. |
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Tuesday November 8 2022 | Philippe Laurent, Aymeric Sauvageon, Jérôme Rodriguez (DAp)
INTEGRAL : 20 ans dans l'espace pour une aventure de plus de 35 ans Summary [click here] Le laboratoire INTErnational d'Astrophysique des Rayons Gamma (INTEGRAL) a été lancé le 17 octobre 2002 de Baikonour (Kasakstan). Depuis lors il est resté sur son orbite elliptique de haute excentricité (environ 3 jours) effectuant 2568 révolutions (au 8 novembre 2022) autour de la Terre pour environ 530 Ms (méga secondes) d'observations scientifiques. Cette mission de taille moyenne transporte deux instruments principaux opérant dans le domaine spectral des rayons X durs et des rayons gamma mous (20keV-10 MeV), dont l'optique est basée sur le concept de masques codés : l'un est dédié à l'analyse spectrale fine avec des capacités d'imagerie modérées (SPI), l'autre est dédié à l'imagerie fine (ou presque) avec des capacités spectrales modérées (IBIS).
Ces 20 ans dans l'espace sont le résultat d'au moins 15 ans de développement du projet avant qu'INTEGRAL puisse dévoiler le mystère du ciel à haute énergie. Le Dap a été profondément impliqué dans cette aventure de longue haleine, depuis le tout début du développement du concept de la mission (grande implication dans les deux instruments, ISGRI et SPI, participation au centre de données, développement de l'analyse s/w, suivi de la caméra) jusqu'à l'analyse actuelle des données en temps réel et des archives, avec un grand nombre d'articles publiés dans tous les domaines permis par les capacités instrumentales et même au-delà. Nous allons, dans ce séminaire, résumer ces plus de 35 ans en présentant divers aspects du projet/de la mission, en nous concentrant particulièrement sur ceux où l'implication du Dap a été cruciale. Nous commencerons par une vue d'ensemble de l'histoire de la mission, des principales caractéristiques instrumentales et des objectifs scientifiques au lancement, puis nous nous concentrerons sur la caméra ISGRI, la couche de détection 20-250 keV du télescope IBIS, qui est suivie au Dap tous les jours. ISGRI a été développée à l'IRFU et a obtenu la plupart des résultats d'INTEGRAL. Nous conclurons par un aperçu rapide et évidemment biaisé de quelques résultats scientifiques obtenus au cours de ces 20 années. Le séminaire sera présenté en Français avec des diapositives en Anglais par Philippe Laurent, Aymeric Sauvageon et Jérôme Rodriguez, bien humblement au nom d'un grand nombre de collègues anciens et actuels. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH VERSION INTEGRAL : 20 years in space and for a 35+ years adventure The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) has been launch on October 17th, 2002 from Baikonour (Kasakstan). Since then it has remained on its high-eccentricity elliptical orbit (about 3 days) performing 2568 revolutions (as of Nov. 8th, 2022) around the Earth for around 530 megaseconds of scientific observations. This medium sized mission caries two main instruments operating in the spectral domain of hard X-rays/Soft Gamma-rays (20keV-10 MeV), whose optics is based on the concept of coded masks: one is dedicated to fine spectral analysis with moderate imaging capabilities (SPI) the other is dedicated to fine(-ish) imaging with moderate spectral capabilities (IBIS). These 20 years in space are the results of at least 15 years of project development before INTEGRAL could unveil the mystery of the high-energy sky. The DAp has been deeply involved in this long-term adventure from the very beginning of the mission concept development (large involvement in both instruments, ISGRI and SPI, participation to the data centre, development of the s/w analysis, monitoring of the camera) to the current analysis of real time and archival data, with a large number of published papers in all fields allowed by the instrumental capabilities and even beyond. We will, in this seminary, summarised these 35 years+ by presenting various aspects of the project/mission, focusing especially on those where the involvement of the Dap has been crucial. This will start with an overall overview of the mission history, the main instrumental characteristics and scientific goals at launch, followed by a focus on the ISGRI camera, the 20-250 keV detector layer of the IBIS telescope, which is followed-up at DAp every days. ISGRI has been developed at IRFU and obtained most of the INTEGRAL results. We will conclude with a quick and obviously biased overview of some scientific results obtained over these 20 years. The talks will be given by Philippe Laurent, Aymeric Sauvageon, and Jérôme Rodriguez humbly on behalf of many, many former and current colleagues and presented in French with slides in English. |
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Tuesday January 10 2023 | Stéphane AUNE, Jean-Charles CUILLANDRE et Pierre ASTIER (CEA/SEDI, CEA/DAp et LPNHE)
20 years of MegaCam scientific operations on the Canada-France-Hawaii Telescope Summary [click here] Over its history CFHT has helped build or hosted dozens of instruments, operating from ultraviolet to mid-infrared wavelengths, attuned to Maunakea’s renowned observing conditions. One instrument, though, eclipses them all – MegaCam. It is in fact hard to imagine CFHT’s success today without the decisions made two decades ago to build what was then the largest digital focal plane in the world. MegaCam took engineering techniques to closely mount CCDs into a single camera to a whole new level. With its 1 degree field of view, MegaCam was able to tile the sky with images, creating rich and precise datasets on enormous scales that could be mined by the entire scientific community for decades. To date MegaCam has led to more peer reviewed science publications than all other instruments used at CFHT combined. It has been used to tune the model of Dark Energy and map vast regions of Dark Matter, unveil faint wisps of stars flung from interacting galaxies, reveal hundreds of free floating planets in star forming regions, and is currently directly contributing to a Stage IV dark energy measurement by providing the key ground-based photometry for the ESA Euclid space mission to derive photometric redshifts of hundreds of millions of galaxies over the northern sky. All of this is part of a revolution in wide field high-resolution optical imaging that CFHT pioneered for many years. Three speakers will offer a complete overview from technical aspects to the most impactful scientific result over these past two decades:
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Tuesday September 12 2023 | Alexander MOISEEV (NASA/GSFC and University of Maryland)
New Mission Concept: Compton Telescope with Coded Aperture Mask, and its Science Perspectives Summary [click here] The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel Explorer-class concept for a next-generation telescope covering the poorly explored hard X-ray and soft gamma-ray energies. This concept builds upon the heritage of past and current missions, improving sensitivity and, very importantly, angular resolution. GECCO uses the combined Coded Aperture Mask and Compton telescope techniques to employ the benefits of both: superior angular resolution provided by the deployable Coded Aperture Mask, and good background rejection and wide field-of-view (FoV) provided by the Compton telescope. It is being developed at NASA/GSFC in collaboration with other US and foreign institutions. GECCO observations will extend arcminute angular resolution to high-energy images of the Galactic plane, combining the
spectral capabilities of INTEGRAL/IBIS and the x-ray imaging of NuSTAR and eROSITA, and will make a bridge to the Fermi-LAT observations, enabling a broad potential for discoveries in the MeV γ-ray sky.
With the unprecedented angular resolution of the coded mask telescope combined with the sensitive, wide FoV Compton telescope, GECCO will focus on two main science objectives:
The GECCO design is based on the novel CZT Imaging calorimeter, which serves as a standalone Compton telescope and as a focal plane detector for the Coded Mask. It also is a powerful tool to measure the γ-radiation polarization. GECCO’s octagon-shaped active shield also serves as a powerful all-sky detector of gamma-ray bursts, prompting the instrument to slew towards the burst direction and localize it with the Coded Aperture’s arcminute accuracy. GECCO will operate in the 100 keV - 10 MeV energy range, with energy resolution of ~ 1% from 0.5 - 5 MeV. The Coded Aperture Mask provides the angular resolution of ∼1 arcmin with a 2 × 2 deg2 fully coded field-of-view, while the Compton telescope provides the angular resolution of 3◦ − 6◦ with a 60 × 60 deg2 field-of-view. The 3σ, 106 s sensitivity is expected to be about 10-5 MeV cm-2 s-1 over the entire energy range. Primary mode of observation is fixed pointing, with extended exposure of the regions of interest. However, as a standalone Compton telescope with wide FoV, Imaging Calorimeter will simultaneously provide wide-area sky exploration, significantly broadening GECCO’s observational scope. |
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Wednesday October 4 2023 | Joint DAp-DPhP seminar Roland BACON (CRAL)
WST - The Wide Field Spectroscopic Telescope Summary [click here] The WST project aim to study and built an innovative 10-m class wide-field spectroscopic survey telescope (WST) in the southern hemisphere with simultaneous operation of a large field-of-view (5 sq. degree) and high multiplex (20,000) multi-object spectrograph facility with both medium and high resolution modes (MOS), and a giant panoramic integral field spectrograph (IFS). The ambitious WST top-level requirements place it far ahead of existing and planned facilities. In just its first 5 years of operation, the MOS will target 250 million galaxies and 25 million stars at medium resolution + 2 million stars at high resolution, and 4 billion spectra with the IFS. WST will achieve transformative results in most areas of astrophysics. The combination of MOS and IFS spectroscopic surveys is one of the key aspects of the project. It is very attractive because of the high complementarity between the two approaches. I will detail this innovative point using the example of the MOS and MUSE surveys performed in the CDFS region. The project aims to be the next major post-ELT project. It is supported by a large consortium of very experienced institutes plus ESO, representing 9 European countries and Australia. |
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Tuesday March 19 2024 | Postdoc seminar Miguel FERNANDES MOITA (LISIS)
The Large Italian X-ray facility (LARIX) and the advancements in Laue lens development Summary [click here] The LARIX (LARge Italian X-ray facility) is a multi-project facility situated in the Scientific-Technological Pole of the University of Ferrara, housed in an underground building featuring a 100-meter-long tunnel with two large experimental rooms on each side. Dedicated to the development and testing of X- and Gamma-ray astronomy instrumentation, LARIX hosts two beamlines: the 12-meter-long LARIX-A in experimental room A, suitable for linearity tests of hard X-ray detectors, reflectivity measurements of X-ray reflector samples, and ground calibrations; and the 50-meter-long LARIX-T installed in the tunnel, ideal for testing gamma-ray reflectors and low-weight gamma-ray detector prototypes when requiring low-divergence beams or lengthy beamlines. This presentation will provide an overview of both installations, their instrumentation, past projects, and opportunities for access through collaborations or transnational access programs like AHEAD. Furthermore, we will discuss recent advancements in Laue lens development that we did in LARIX, particularly the TRILL project, supported by ASI, aimed at advancing the technological readiness of Laue lenses. Future goals include the ASTENA mission concept, submitted to the ESA program 'Voyage 2050', featuring a narrow field telescope (NFT) based on a focusing Laue lens with an energy passband from 50 to 700 keV and a 20-meter focal length, promising breakthroughs in sensitivity and angular resolution in this energy band. Additionally, we will explore innovative concepts such as a high-energy Laue lens spectro-polarimeter, inspired by the recent success of the IXPE mission. |
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Tuesday April 23 2024 | Florent MERTENS (Observatoire de Paris)
Unveiling the Physics of the Cosmic Dawn and the Epoch of Reionisation in the SKA era Summary [click here] Direct observation of the Cosmic Dawn and Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are underway aiming at statistically detecting the 21-cm signal fluctuations from the EoR and CD, and paving the way for the SKA EoR CD experiment which will be capable of directly image the large-scale neutral hydrogen structures from these distance epochs. In this talk, I will present recent results from the LOFAR-EoR and NenuFAR Cosmic Dawn collaborations. After publishing the deepest upper limit on the signal power-spectra at z~9, which has made it possible to set constraints on the physics of the IGM during the EoR, the LOFAR-EoR team is progressing towards a deeper upper limit on a broader range of redshift. On the Cosmic Dawn front, the NenuFAR CD team recenty published a first upper limit on the signal power-spectra at z~20. These new results will be discussed, as well as the improvements in instrument calibration and foreground mitigation that enabled these achievements. |
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Tuesday June 25 2024 | Jérôme RODRIGUEZ, Bertrand CORDIER & Damien TURPIN (DAp)
The SVOM Launch: En route for the transient hunt Summary [click here] The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a Sino-French mission planned to be launched on June 22nd 2024 from Xichang China. The launch of the satellite opens the time when scientists will step up and start exploiting the data. SVOM, however, is already a long-term project during which the involvement of engineers, developers, and scientists has already been huge. In this presentation I will overview some of the aspects of the mission, the scientific programs and aims of the project, but also recall its history and try and acknowledge the involvement of many colleagues which permitted the achievement of this fantastic machine. I will also introduce some of the novelties and specificities of the mission, in particular all the aspects dedicated to the time domain astronomy and real time analysis, implying the development of a dedicated science center, specific tools, and shifts for a certain of us as burst advocates. Finally we will (try to) have a zoom with those of us that have attended the launch in China, so that they can share with us their feeling of the launch and first impressions post launch, in this opening scientific exploitation era.
The talk will be given in French with slides in English. |
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Tuesday May 17 2022 | Recent hiree seminar Benjamin MAGNELLI (DAp)
A decade of the main-sequence of star-forming galaxies: New insights and perspectives on massive galaxy formation Summary [click here] The observed tight correlation between the star formation rate (SFR) and the stellar mass of star-forming galaxies (SFGs) is now well constrained over the last 10 Gyr of look-back time. This so-called main sequence (MS), whose normalization declines from z~3 to 0, is commonly interpreted as evidence that SFGs are evolving primarily through a steady and long star-forming mode, likely sustained by the cold gas accretion along the cosmic web. Over the last decade, a plethora of studies have investigated within this framework the physical properties of SFGs along and across the MS, establishing key scaling relations between, e.g., the stellar mass, gas content, and/or morphology of SFGs in the SFR-stellar mass plane. In this talk I will review past and recent observational evidences of this new MS paradigm and how it has shaped our understanding of the evolution of massive galaxies. Then, I will present the limitations of this simple paradigm, and in particular how it fails to explain the more diverse than anticipated population of MS galaxies (e.g., starburst hidden within the MS), the importance of secondary parameters (e.g., environment) and the transition of SFGs to quiescence. I will conclude by presenting future observational opportunities that can be used to investigate this hidden complexity within the main sequence and to further unveil the physics involved in the evolution of massive galaxies over cosmic time. |
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Tuesday November 15 2022 | Large project seminar Anaëlle MAURY (LFEMI)
Magnetic fields from star-forming cores to protostellar disks: a review of major contributions from the MagneticYSOs project Summary [click here] Wherever we have the means of observing them, magnetic fields are detected across the full spectrum of astrophysical environments, from our own Earth, to stars, and cosmological structures. Magnetic fields are also present at all scales and evolutionary stages of star-forming structures. They have long been suspected to play a key role in shaping the typical outcome of the star formation process, such as stellar mass, spin, and multiplicity, or even the fate of stars towards their ultimate stages.
In this talk, I will provide a global outlook on the progresses made in the recent years to characterize the role of magnetic fields during the embedded phases of the star formation process. Thanks to the development of observational capabilities and the parallel progress in numerical models capturing most of the important physics at work during star formation, the MagneticYSOs team successfully confronted detailed predictions of magnetized models to observational properties of the youngest protostars. I will present the physical processes and observational methods allowing to trace the magnetic field in embedded protostars, and review the main steps, success and limitations in comparing real observations to synthetic observations from the non-ideal MHD models. I will show how our work has shed light on the physical conditions required to ensure an efficient magnetic field coupling, and present unexpected results regarding the two main agents responsible for the coupling in star-forming cores: dust grains and ionized gas. Following this Ariane thread, I will argue our observational and theoretical findings support a novel scenario where the angular momentum problem for star formation may be actually “solved” not by the formation of large protoplanetary disks but by the combination of 1) lack of organized rotation motions at large envelope radii, 2) the inefficient angular momentum transport due to magnetic braking in the inner envelope (and angular momentum removed through rotating outflows generated by the presence of the magnetic field), and 3) a local origin of the angular momentum incorporated in the star–disk system. Reference review |
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Friday November 18 2022 | Group seminar Camille DIEZ (University of Tübingen)
Using Vela X-1 to understand accretion and wind structure in neutron star High-Mass X-ray Binaries (HMXBs) Summary [click here] The spectral and timing behaviour of HMXBs offers a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. The bright and persistent neutron star HMXB Vela X-1 is one of the key systems for such studies with both current and future instruments. It has a complex clumpy stellar wind, prominent cyclotron resonant scattering features (CRSFs) and strong flares. Understanding the variability of the systems on both short time scales of a few hundreds seconds and along its 9d orbit with current instruments enables us to make predictions for future observations with XRISM and Athena and to devise the best observational strategy for Vela X-1. Here, we analyse two new observations taken with NuSTAR and XMM-Newton at orbital phases ~0.5 and ~0.75 and follow the evolution of spectral parameters down to the pulse period (~300s) time-scale. The flux-dependency we observe in the spectral shape implies a change in the properties of the Comptonising plasma and the observed drop of the CRSF energy following a strong flare may indicate a change in the accretion geometry. The strong variability of absorption is due to the presence of a large-scale wind structure, such as accretion- and photoionisation wakes, combined with the variable line of sight as the neutron star moves along the orbit. In particular, we, for the first time, are able to trace the onset of the wakes with high time resolution and compare to predictions from simulations. |
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Tuesday November 22 2022 | Recent hiree seminar Antonio GARCIA MUÑOZ (DAp)
Exoplanet atmospheres: An anticipated revolution Summary [click here] The launch of the JWST less than a year ago is expected to set a turning point in exoplanet science, which is progressively transitioning from detection and population statistics to in-detail characterization of the exoplanets’ atmospheres. In this talk, I will present an admittedly biased
perspective of what this may represent for our understanding of exoplanets, and how ongoing theoretical work and future telescopes may build upon JWST’s legacy. |
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Friday November 25 2022 | Special seminar Leïla GODINAUD, Anne DECOURCHELLE, Fabio ACERO (DAp)
De Nova Stella : Séminaire d'anniversaire des 450 ans de la supernova de Tycho Summary [click here] En novembre 1572, l'observation de l'apparition d'une "nouvelle étoile" dans le ciel et les observations détaillées réalisées par Tycho Brahe ont transformé la façon dont nous voyons et comprenons notre Univers. Pour marquer l'événement, nous proposons un séminaire en trois parties allant de l'observation historique de l'événement jusqu'aux connaissances actuelles sur la supernova SN1572 et son vestige.
La conférence sera suivie d'un café gourmand dans le hall de la salle Galilée. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH De Nova Stella: 450th anniversary seminar for Tycho's supernova In November 1572, the observation of the apparition of a "new star" in the sky and the detailed observations made by Tycho Brahe transformed the way we see and understand our Universe. To mark the event we propose a three-part seminar ranging from the historical observation of the event to the current knowledge about the supernova SN1572 and its remnant. The conference will be followed by a gourmet coffee in the hall of the Galilee room. |
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Tuesday November 29 2022 | Elisa COSTANTINI (SRON, Netherlands)
The cold and dusty medium in our Galaxy as seen in the X-rays Summary [click here] X-rays have unique advantages in studying absorption and scattering from interstellar dust. For example, sharp and deep absorption features of Mg, Si, O and Fe, which are the building blocks of silicates, fall in the X-ray band. Present X-ray observatories already delivered to us interesting results, challenging the common paradigm on interstellar dust chemical and physical characteristics. Future instruments will open up an unexplored window, revealing the most dense environments of our Galaxy.
In this talk I will illustrate the state-of-art of our understanding of dust as seen in the X-rays, and future prospects, using for example, the upcoming XRISM satellite. |
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Tuesday January 10 2023 | Stéphane AUNE, Jean-Charles CUILLANDRE et Pierre ASTIER (CEA/SEDI, CEA/DAp et LPNHE)
20 years of MegaCam scientific operations on the Canada-France-Hawaii Telescope Summary [click here] Over its history CFHT has helped build or hosted dozens of instruments, operating from ultraviolet to mid-infrared wavelengths, attuned to Maunakea’s renowned observing conditions. One instrument, though, eclipses them all – MegaCam. It is in fact hard to imagine CFHT’s success today without the decisions made two decades ago to build what was then the largest digital focal plane in the world. MegaCam took engineering techniques to closely mount CCDs into a single camera to a whole new level. With its 1 degree field of view, MegaCam was able to tile the sky with images, creating rich and precise datasets on enormous scales that could be mined by the entire scientific community for decades. To date MegaCam has led to more peer reviewed science publications than all other instruments used at CFHT combined. It has been used to tune the model of Dark Energy and map vast regions of Dark Matter, unveil faint wisps of stars flung from interacting galaxies, reveal hundreds of free floating planets in star forming regions, and is currently directly contributing to a Stage IV dark energy measurement by providing the key ground-based photometry for the ESA Euclid space mission to derive photometric redshifts of hundreds of millions of galaxies over the northern sky. All of this is part of a revolution in wide field high-resolution optical imaging that CFHT pioneered for many years. Three speakers will offer a complete overview from technical aspects to the most impactful scientific result over these past two decades:
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Tuesday April 11 2023 | Anne VERHAMME (Université de Genève)
Searching for the sources of Cosmic Reionisation Summary [click here] Over the past 5 years, we have made tremendous progress on both direct detections of the escape of ionizing radiation from galaxies, over a broad range of redshifts (and instruments), and the tests and validations of indirect probes of the escape of ionizing radiation from galaxies, both from observations and simulations. I will review these recent achievements, and describe the next steps to understand the nature of the sources of reionisation. |
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Tuesday April 18 2023 | Judith IRWIN (Queen's University, Canada)
CHANG-ES — Past and Future Summary [click here] CHANG-ES (Continuum Halos in Nearby Galaxies — an EVLA Survey) is a project to observe 35 nearby galaxies that are edge-on to the line of sight to focus on their radio halos and the disk-halo connection. Wide-band VLA observations at L-band (1.5 GHz) and C-band (6.0 GHz) have provided opportunities to study in-band spectral indices, and observations in all four Stokes parameters with Rotation Measure Synthesis has led to a new understanding of the structure of kpc-scale magnetic fields in disk galaxies. This talk will highlight some of the results of the project and look to the future, as newly completed S-band (3.0 GHz) observations have filled in the L-band to C-band gap and led to the widest contiguous frequency coverage yet seen for galaxies. |
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Tuesday June 6 2023 | Postdoc seminar Carlos GÓMEZ GUIJARRO (LCEG)
Understanding the role of compact star formation in galaxy evolution with ALMA and JWST Summary [click here] Massive elliptical galaxies in the local universe appear to have their high-redshift analogs in the form of extremely compact quiescent galaxies. Therefore, it seems that compact star formation appears to play a pivotal role in the evolutionary pathways of massive galaxies across cosmic history. However, it remains to be understood what this role is in the broader picture set by the main sequence and the scaling relations in galaxy evolution. From an ALMA survey at 1.1mm, we reveal that compact star formation appears to be the norm in massive star-forming galaxies, and sizes as extended as typical star-forming stellar disks are rare. A population of galaxies with modest star formation rates, but which exhibit extremely compact star formation with starburst-like depletion timescales unveils. Compact star formation appears as a physical driver of depletion timescales, gas fractions, and dust temperatures. Gas and star formation compression seems to be a mechanism that allows to hold their star formation rate even when their gas fractions are low and they are presumably on the way to quiescence. Another population of galaxies missed in the deep optical surveys but bright at far-IR/mm wavelengths unveils thanks to recent JWST observations. We present a study investigating the drivers of dust attenuation in massive galaxies in the JWST-era, showing how the stellar mass and morphology plays an important role, with evidence for more compact stellar profiles resulting in the obscuration of galaxies. |
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Thursday June 29 2023 | Julien DE WIT (Department of Earth, Atmospheric and Planetary Science, MIT, USA)
Robustly Characterizing Exoplanets in a New Era of High-Precision Transit Measurements Summary [click here] With a new-generation of great observatories coming online this decade, unprecedented insights into exoplanets will soon be within reach. Observatories such as the James Webb Space Telescope (JWST) notably enable the study of atmospheres around terrestrial exoplanets and can reveal tri-dimensional structures in the atmospheres of their larger counterparts. Robustly leveraging new observations to reach such achievements will however require extra care as the models currently used may not be up to par with their precision.
During this presentation, I will introduce work done by MIT’s Disruptive Planets group and collaborators towards supporting the robust in-depth characterization of exoplanets. I will specifically discuss how not accounting for the true shape of a planet can lead to a misinterpretation of its interior properties as well as atmospheric structure; how the current state of our understanding of light-matter interactions can similarly affect our interpretation of planetary spectra and thus inferences regarding their atmospheric properties; and how the current state of emission spectrum models for stars may even prevent from disentangling between the contribution of a planet and its host star, to start with. I will also present possible ways to address these challenges. I will end with a step-by-step roadmap to the robust characterization of temperate terrestrial planets with JWST, which includes habitability assessment. |
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Tuesday September 5 2023 | Postdoc seminar Lucie BAUMONT (LCS)
Galaxy Cluster Cosmology with fgas Summary [click here] The current Standard Model of Cosmology has successfully explained many phenomena, but it predicts that the majority of the Universe consists of dark matter and dark energy, whose properties are poorly understood. Because huge volumes collapse to form galaxy clusters, the largest known gravitationally bound structures, they are an ideal laboratory to study the Dark Universe. In fact, the ratio of baryonic matter to total matter in a massive cluster, fgas, can be considered representative of the matter content of the Universe as a whole. Measurements of fgas from the heaviest, dynamically relaxed galaxy clusters place powerful constraints on cosmological parameters as well as the dark energy equation of state. I will discuss constraints derived from fgas measurements using a multi-wavelength set of X-ray and optical data and provide outlook on the future of this measurement in the age of precision cosmology. |
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Tuesday September 19 2023 | Arjen VAN DER WEL (Ghent University, Belgium)
Galaxy Evolution from z=3 to the Present Day: Recent Insights from JWST and VLT surveys Summary [click here] Thanks to an abundance of excellent observational data the evolution of the population of galaxies in terms of its stellar mass function, the distribution of star-formation across galaxies and their structural properties is known with good precision. The challenge we face, though, is to decipher the evolutionary paths of individual galaxies. Two roads to achieving this goal are 1) to reconstruct star-formation histories of individual galaxies through detailed modeling of high-quality spectra, and 2) attempt to define 'rules' for galaxy evolution that tell us how the collection of individual galaxies must evolve so that the population properties are reproduced. I will describe our recent progress along these lines, enabled by high-quality data from VLT spectroscopy of z~1 galaxies (the LEGA-C survey) and high-quality imaging data from HST (CANDELS) and JWST (CEERS). |
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Tuesday October 3 2023 | Postdoc seminar Lara PANTONI (LFEMI)
Dust millimetre emission in nearby galaxies with NIKA2 (IRAM-30m): major challenges and latest results of the IMEGIN Large Program Summary [click here] The millimetre part of the spectrum is one of the least explored parts of a galaxy’s spectral energy distribution (SED), yet it contains emissions from three fundamentally important physical processes. These processes are thermal emission from dust, free-free emission from ionized gas and synchrotron emission from relativistic charged particles moving in the galactic magnetic field. The NIKA2 camera (IRAM-30m telescope), observing at 1.15 mm and 2 mm, provides additional data points for input into the comprehensive SED models and allows us to:
During my presentation, I will focus on the major challenges linked with data processing, uncertainty propagation, and large-scale emission filtering in NIKA2 maps (due to atmosphere removal during the data reduction process). I will show and discuss the latest significant results on NGC891 (Katsioli et al. 2023); NGC4254 (Pantoni et al. in prep.); NGC2976 and NGC2146 (Ejlali et al. in prep.); millimetre morphology (Nersesian et al. in prep.); future perspectives/applications. |
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Tuesday October 17 2023 | Postdoc seminar Sudip CHAKRABORTY (LEPCHE)
Unraveling the magnetic origin of dense non-equatorial ultrafast outflows in black hole X-ray binaries Summary [click here] Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era. |
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Tuesday November 28 2023 | Elisa CHISARI (University of Oxford)
A rising tide: galaxy alignments in the Stage IV era Summary [click here] A new generation of galaxy surveys is at our doorstep. By mapping billions of galaxies over the sky, they will deliver precision constraints on the cosmological model. The significant gains in constraining power are accompanied by the need to develop more accurate models of galaxy shapes. Several physical effects will come into play that could be neglected or simplified in previous analyses. I will discuss the constraining power of upcoming surveys, focusing mostly on Rubin Observatory, and the modelling challenges they face. Amongst such challenges, I will focus mostly on the “intrinsic alignments” of galaxies: alignments between their shapes that are likely to arise from tidal interactions between them. I will describe some new modelling strategies that could help reduce their impact on cosmological constraints. Finally, I will present some opportunities that arise for using intrinsic alignments themselves as a probe of cosmology and galaxy evolution. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Wednesday December 6 2023 | Ute LISENFELD (University of Granada)
Molecular gas in galaxies in compact groups Summary [click here] Galaxies in compact groups live in an environment where constant interactions take place, both through tidal and through collisional processes. They are are therefore ideal objects to study the influence of the environment on galaxy properties and evolution, in particular the quenching of star formation.
The atomic hydrogen is extremely affected by this environment, with high deficiencies in many cases, whereas the molecular gas content is on average relatively normal in most galaxies. However, the properties of the molecular gas are drastically different if one selects those galaxies, based on their mid-infrared colours from the Spitzer and WISE satallites, that are in transitioning phase from active to quiescent which have have a considerably lower molecular-to-stellar mass and star formation efficiency. In my talk I will present result form observations of the molecular gas and star formation in galaxies in compact groups, both from single dish and interferometric observations, that show indications that in many cases the molecular is highly perturbed (and therefore unable to form stars) in compact groups. |
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Tuesday December 12 2023 | Stefano ANDREON (INAF-OA Brera, Milan, Italy)
First massive galaxy clusters emerging from the cosmic web at z~2 Summary [click here] In this talk I report upon our results on the intracluster medium (ICM) of two clusters at the time when first clusters start to emerge from the cosmic web, z~2. Results are derived from new, high resolution, deep SZ and X-ray data providing us with the measurement of the two most distant resolved pressure profiles. IDCSJ1426 cluster at z=1.75 has a core whose properties are not far from the final stage, while the remaining part of the cluster is experiencing a sizable gas, heat and entropy transfer. JKCS041 at z=1.80 is caught just after a major merger event as evidenced by its SZ-X-ray peak offset, its low central pressure, and its low Compton-Y parameter compared to its WL mass. Comparison with plausible descendents shows that its ICM will experience major changes at all radii. |
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Tuesday January 23 2024 | Francisca KEMPER (Institute of Space Sciences (ICE-CSIC) / ICREA / IEEC, Spain)
The interstellar dust reservoir in galaxies Summary [click here] The evolution of interstellar dust reservoirs, and the evolution of galaxies themselves go hand-in-hand, as the presence of dust alters evolutionary drivers, such as the interstellar radiation field and the star formation history, while at the same time, the dust is being formed and altered by processes taking place in galaxies. However, far-infrared and submillimeter studies have revealed enormous dust masses at high redshifts that are difficult to explain with dust production from evolved stars (the so-called "dust budget problem"), while in the nearby universe there is also a significant mismatch between the dust production rate and the dust mass observed in the interstellar medium of galaxies. I will go over some possible explanations in an attempt to find a way forward towards a solution to this seeming discrepancy. |
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Tuesday January 30 2024 | Florian PACAUD (University of Bonn)
Galaxy clusters and filaments in the first eROSITA all-Sky survey Summary [click here] Launched in July 2019, eROSITA was set to survey the sky in the soft X-ray band for four years and provide a much awaited update to the old ROSAT all-sky survey. In a few weeks, the collaboration will release the first all-sky map obtained from the first six month of observation. In this talk, I will present the instrument and its technical abilities. I will then explore the all-sky map and show selected results on individual local clusters and filaments. Finally, I will give a preview of the soon-to-be-released data products, including AGN and galaxy catalogs, and present some of the methods used for their cosmological analysis. |
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Tuesday February 27 2024 | Hakim ATEK (IAP)
Unveiling the faintest and the brightest galaxies at early times with JWST Summary [click here] The JWST is revolutionizing our understanding of the early Universe by unveiling a wealth of bright galaxies at z>9 and faint AGNs at z>5. I will present the latest constraints on the overabundance of UV-bright galaxies at z>9, which is 10-100 times higher than galaxy formation models. I will discuss to what extent recent theoretical efforts can reproduce such observations, and how future wide-area surveys such as Euclid will help put stronger constraints ion the bright-end of UVLF at z>8. On the other hand, faint galaxies, representing the building blocks of present-day galaxies, have eluded spectroscopic constraints, even with the deepest JWST campaigns so far. I will present the results of our UNCOVER survey, which combines ultra-deep NIRSpec spectroscopy with the strong lensing magnification of A2744 cluster. We characterize ultra-faint galaxies with intrinsic absolute magnitude between Muv=-17 and Muv=-15 at 6<z<8, and stellar masses down to 10^6 solar masses. I will discuss our plans to obtain the deepest observations on sky with the GLIMPSE program to the faintest galaxy population out to z=15 and beyond. |
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Tuesday March 5 2024 | Recent hiree seminar Emmanuel BERTIN (LCEG)
Wide-field imaging meets deep learning: new challenges, new opportunities Summary [click here] Since the 19th century, wide-field imaging has significantly advanced numerous fields of Astrophysics, spanning from the study of solar system bodies to observational cosmology. Ongoing and future optical/near-infrared imaging surveys face many interesting data analysis challenges, especially in time domain astronomy, in a context where detector technology enable wide-field observations at increasingly high frame rates. Through concrete examples, I will show with how Deep Learning techniques offer promising solutions to address such challenges, and provide new scientific opportunities. |
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Tuesday April 2 2024 | James DUNLOP (University of Edinburgh)
Charting the emergence of the first galaxies with JWST Summary [click here] The James Webb Space Telescope (JWST) is transforming our view of galaxy formation and evolution in the young Universe. I will provide an overview of the latest results from the PRIMER survey, the largest JWST Cycle-1 “Galaxies” programme which, in combination with other public JWST imaging, is now enabling us to chart the early growth of the galaxy population back to within ~300 million years of the Big Bang. Specifically, I will present and discuss the first robust determination of the evolving galaxy luminosity function extending out to redshifts z~13, as well as new measurements of the galaxy stellar mass function reaching out to z~9. I will then attempt to interpret these results in the context of our current understanding of the evolving dark matter halo mass function and the efficiency with which galaxies are able to convert their baryons into stars. Finally, I will discuss the prospects for future progress with JWST and with the Atacama Large Millimeter Array (ALMA), including the possibility of uncovering the emergence of the first galaxies. |
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Tuesday April 23 2024 | Florent MERTENS (Observatoire de Paris)
Unveiling the Physics of the Cosmic Dawn and the Epoch of Reionisation in the SKA era Summary [click here] Direct observation of the Cosmic Dawn and Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are underway aiming at statistically detecting the 21-cm signal fluctuations from the EoR and CD, and paving the way for the SKA EoR CD experiment which will be capable of directly image the large-scale neutral hydrogen structures from these distance epochs. In this talk, I will present recent results from the LOFAR-EoR and NenuFAR Cosmic Dawn collaborations. After publishing the deepest upper limit on the signal power-spectra at z~9, which has made it possible to set constraints on the physics of the IGM during the EoR, the LOFAR-EoR team is progressing towards a deeper upper limit on a broader range of redshift. On the Cosmic Dawn front, the NenuFAR CD team recenty published a first upper limit on the signal power-spectra at z~20. These new results will be discussed, as well as the improvements in instrument calibration and foreground mitigation that enabled these achievements. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Tuesday April 30 2024 | Postdoc seminar Dinil PALAKKATHARAPPIL (LDE3)
Precise Stellar Age Determination of Stars with Gaia and TESS: Prospects and Insights for the upcoming PLATO and HAYDN Missions Summary [click here] Determining precise star ages is essential for insights into stellar, exoplanetary, and galactic evolution, but it remains a complex challenge. The two most precise techniques are (1) asteroseismology, which is the study of the internal structure of stars by means of intrinsic global oscillations, and (2) analysis of stellar clusters where stars are formed from the same molecular cloud at the same time, and the observed colour-magnitude
diagram is a strong diagnostic of the cluster age. In this talk, I will combine these two techniques to determine the age of intermediate age open cluster NGC 2477. We used astrometry and colour from Gaia to extract membership probabilities and colour information about the stars in clusters, while pulsating stars were found using TESS data. Observations are complemented with asteroseismic modelling using MESA and GYRE to obtain the precise age of the cluster. In the second part of the talk, I will detail my contribution to the development of the ground segment of the ESA M3 PLATO mission to obtain stellar mass, radius, and age as well as several rotation and magnetic activity indicators for which CEA is responsible. I will conclude by showcasing some applications of the PLATO pipeline using Kepler and TESS data. |
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Tuesday May 7 2024 | Thomas HENNING (Max Planck Institute for Astronomy, Heidelberg)
Towards New Horizons: Star and Planet Formation with JWST Summary [click here] The James Webb Space Telescope (JWST) is the most powerful space-based infrared observatory ever built. The first part of the talk will discuss the main structures of the telescope and its assembly after launch. The talk will then highlight the extremely exciting science results in the field of star and planet formation, including the nature of protostars and planet-forming disks. A special emphasis will be placed on the accretion process and the chemical building blocks of disks and what we can learn from infrared spectroscopy regarding the inventory of material in the terrestrial planet-forming zone. |
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Tuesday May 14 2024 | Recent hiree seminar Damien TURPIN (DAp)
The high-energy and multi-messenger transient sky in the eyes of the SVOM mission Summary [click here] Over the past decades, the study of highly energetic transient phenomena has led to several scientific breakthroughs from the discovery of r-process nucleosynthesis production sites to the first credible identification of ultra-high-energy cosmic-ray accelerators. In a general manner, these explosive events allow us to study extreme physical regimes (extreme gravity, extreme magnetic fields, relativistic shocks) and are unique probes to study the Universe properties up to high redshift. In the last 10 years, an unprecedented amount of new generation instruments observing the sky at all wavelengths and with all astrophysical messengers have significantly contributed to enlarging our vision of the transient sky phenomena. This observational revolution initiated by the gravitational wave interferometers has forced the astrophysical community to rapidly adapt to the new challenges of multi-messenger astronomy. In June 2024, the French community will have a leading role in this young research domain with a strong implication for the French lab. in the upcoming SVOM mission. In this seminar, I will review some of the latest results obtained by the scientific community exploring the high-energy transient sky, especially with a focus on the sources of interest for the SVOM mission. I will show how important will be the SVOM mission contribution to this exciting field of research that has probably revealed only a subset of its richness. |
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Tuesday May 28 2024 | Paul GOLDSMITH (JPL)
Velocity-Resolved Fine Structure Line Observations and Star Formation: New Results and New Capabilities Summary [click here] What controls star formation? is a key question in astrophysics, and one very important aspect of this is the interaction of newly-formed stars with their surroundings. The radiative and mechanical feedback from young, massive stars can be dramatic. The altered composition and
increased temperature that result make atomic and ionic fine structure lines ideal probes of stellar feedback. The value of such observations is dramatically increased if the spectral lines are velocity-resolved so that the momentum and energy impact on the stars’ surroundings can be determined. But such observations must be carried out from suborbital or space observatories due to absorption in the Earth’s atmosphere. The fine structure lines of ionized
carbon, and atomic oxygen are the most important and have been widely observed with high spectral resolution starting with the Herschel HIFI instrument and continuing with upGREAT on SOFIA. In this talk I will discuss some recent fine structure line observations focusing on the effects of star formation on the surrounding interstellar medium and possible problems with measuring the rate of star formation. I will conclude by presenting two fine structure line spectroscopic balloon missions. GUSTO was launched on 31 December 2023, and until 27 February 2024 surveyed the 205 μm line of [NII] and 158 μm line of [CII] in the central portion of the Milky Way and the Large Magellanic Cloud. ASTHROS, which is to follow at the end of 2024, has a much larger 2.5m diameter telescope, and will observe both [NII] fine structure lines to derive the electron density in selected regions. |
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Tuesday June 11 2024 | Mustapha MEFTAH (LATMOS)
Les NanoSatellites au service de l’observation de la Terre et de l’univers Summary [click here] Le thème de la conférence portera sur l’observation de la Terre, le climat, les satellites et le NewSpace. Le 24 janvier 2021, une fusée Falcon 9 était lancée avec succès par SpaceX depuis Cap Canaveral, en Floride. Sous la coiffe du lanceur, il y avait 143 satellites. C’est à ce jour le plus grand nombre de satellites jamais déployés en une seule mission. Uvsq-Sat, premier nanosatellite français dédié à l’observation de variables climatiques essentielles, faisait partie de cette épopée. Depuis plus de trois ans, Uvsq-Sat observe la Terre et le Soleil de manière assidue. A 550 km d’altitude, ce satellite mesure la quantité d'énergie solaire réfléchie vers l'espace et la quantité d'énergie thermique émise par la Terre vers l’espace.
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Tuesday June 25 2024 | Jérôme RODRIGUEZ, Bertrand CORDIER & Damien TURPIN (DAp)
The SVOM Launch: En route for the transient hunt Summary [click here] The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a Sino-French mission planned to be launched on June 22nd 2024 from Xichang China. The launch of the satellite opens the time when scientists will step up and start exploiting the data. SVOM, however, is already a long-term project during which the involvement of engineers, developers, and scientists has already been huge. In this presentation I will overview some of the aspects of the mission, the scientific programs and aims of the project, but also recall its history and try and acknowledge the involvement of many colleagues which permitted the achievement of this fantastic machine. I will also introduce some of the novelties and specificities of the mission, in particular all the aspects dedicated to the time domain astronomy and real time analysis, implying the development of a dedicated science center, specific tools, and shifts for a certain of us as burst advocates. Finally we will (try to) have a zoom with those of us that have attended the launch in China, so that they can share with us their feeling of the launch and first impressions post launch, in this opening scientific exploitation era.
The talk will be given in French with slides in English. |
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Tuesday October 1 2024 | Recent hiree seminar Elsa DUCROT (LDE3)
Detection and characterization of rocky planets around ultra-cool stars Summary [click here] The launch of the James Webb Space Telescope (JWST) marked a significant milestone in the field of exoplanet research. For the first time, we can study the atmospheres of Earth-sized rocky planets—an opportunity that was previously unattainable. However, this is currently possible only around the coolest stars, known as red dwarfs. These stars are ideal targets for several reasons: (1) their smaller size makes it easier to detect and study small, transiting planets; (2) their lower luminosity leads to more frequent planetary transits for the same level of stellar irradiation; (3) they are the most abundant stars in the galaxy, and (4) planets orbiting them are more likely to be rocky planets with short orbital periods. In the near future, only such planetary systems are expected to produce signal-to-noise ratios (SNRs) high enough to confidently confirm or rule out the presence of atmospheres on potentially habitable planets.
In this context, my research focuses on: (1) detecting new rocky, temperate planets around red dwarfs using the SPECULOOS telescopes, and (2) observing these planets with JWST to assess the presence and composition of their atmospheres or determine the nature of their surfaces. In this talk, I will share insights into these areas of my work, including the discovery of the new SPECULOOS-3b exoplanetary system and the detailed characterization of TRAPPIST-1's innermost planets using JWST data. I will also discuss stellar contamination caused by photometric heterogeneities, which represents the primary challenge in characterizing planets around red dwarfs in transit, along with various strategies to overcome this obstacle. Finally, I will offer some perspectives on the future study of rocky planets orbiting red dwarfs. WARNING: the first minutes of the recording are missing. |
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Tuesday November 5 2024 | Special seminar Jean-Marc BONNET-BIDAUD (DAp)
Chine, la civilisation du ciel transitoire Summary [click here] En Chine, dès l'époque classique des Han (IIe siècle AEC), le Ciel est conçu comme le miroir de la Terre, lieux d'évènements fortuits (éclipses, comètes, taches solaires, novae et supernovae,..) qui sont traqués et catalogués par une armée de scientifiques au service de l'empereur. De nombreux documents astronomiques ont ainsi été produits sur plusieurs millénaires, conservés ou récemment redécouverts par l’archéologie. C’est le cas notamment de la plus ancienne carte d’étoiles connue au Monde conçue il y a plus de 1200 et préservée dans un monastère bouddhique de la route de la Soie.
Avec la présentation de ces documents, on retracera les étapes, les instruments et les concepts qui ont mené l'astronomie chinoise ancienne à de nombreuses découvertes du ciel transitoire, à des époques où l’Europe n’envisageait encore le Ciel que comme une voute céleste fixe, éternelle et immuable. Avec ce recul de l’histoire, la mission franco-chinoise SVOM, qui va détecter et cataloguer les plus puissants phénomènes célestes variables, apparait aujourd'hui comme une continuation logique dans laquelle la France rejoint symboliquement la Chine dans la préoccupation du ciel transitoire. |
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Tuesday November 12 2024 | Joel ONG (Institute for Astronomy, University of Hawaii at Manoa)
New Views into Stellar Rotation from Asteroseismology Summary [click here] Asteroseismology — the analysis and interpretation of stellar oscillations — remains the only means by which we may directly inspect the properties of stellar interiors. I will describe new theoretical developments regarding the interpretation of asteroseismic rotational signatures in a particular class of variable stars — red-giant solar-like oscillators. While waves in most variable stars propagate in a single mode cavity, I formulate a description of multicavity oscillations in these red giants by way of a physical analogy where they behave as "acoustic molecular orbitals". I describe two recent observational results which this formalism has helped us to interpret: 1. the detection of engulfment signatures in evolved red giants, and 2. direct constraints on the misalignment angle between the rotational axis of the core and of the envelope, in the central star of a spin-orbit-misaligned planetary system. |
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Tuesday November 19 2024 | Postdoc seminar Jose Ignacio "Nacho" ANEZ LOPEZ (LFEMI)
Signatures of magnetic braking in Class 0 protostars ? Exploring the gas kinematics in magnetized models of low-mass star formation Summary [click here] The role of magnetic braking in regulating gravitational collapse and circumstellar disk during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disk sizes and the launching of high-velocity collimated jets, we aim at more direct tests of the magnetic braking in observations. The study of polarized dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process.
In the present work, we have used both non-ideal MHD models and synthetic observations from the radiative transfer of protostellar formation to put constraints on the magnetically-regulated disk formation scenario. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetized model shows a higher angular momentum redistribution above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O(2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetized model similar to the characteristics found in observational work. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from radial motions. |
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Tuesday October 11 2022 | Françoise COMBES (LERMA)
Overview of Cosmology with SKA, the Square Kilometer Array Summary [click here] SKA is a new technology radio-telescope array, about two orders of magnitude more sensitive and rapid in sky surveys than present instruments. It will be able to detect and measure the redshifts of billions of galaxies at the redshifts up to z=2, to probe through baryonic acoustic oscillations the nature of dark energy; it will probe the cosmic dawn of the universe, just afer recombination, and during the epoch of reionisation (z=6-15); it will be the unique instrument to map the atomic gas in high redshift galaxies, and determine the amount and distribution of dark matter in the early universe. With SKA-VLBI, it will unveil the accretion and feedback processes near super-massive black holes, and results from precursors will be shown. We will discuss these exciting perspectives, which will concretize at the end of the decade. |
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Tuesday November 15 2022 | Large project seminar Anaëlle MAURY (LFEMI)
Magnetic fields from star-forming cores to protostellar disks: a review of major contributions from the MagneticYSOs project Summary [click here] Wherever we have the means of observing them, magnetic fields are detected across the full spectrum of astrophysical environments, from our own Earth, to stars, and cosmological structures. Magnetic fields are also present at all scales and evolutionary stages of star-forming structures. They have long been suspected to play a key role in shaping the typical outcome of the star formation process, such as stellar mass, spin, and multiplicity, or even the fate of stars towards their ultimate stages.
In this talk, I will provide a global outlook on the progresses made in the recent years to characterize the role of magnetic fields during the embedded phases of the star formation process. Thanks to the development of observational capabilities and the parallel progress in numerical models capturing most of the important physics at work during star formation, the MagneticYSOs team successfully confronted detailed predictions of magnetized models to observational properties of the youngest protostars. I will present the physical processes and observational methods allowing to trace the magnetic field in embedded protostars, and review the main steps, success and limitations in comparing real observations to synthetic observations from the non-ideal MHD models. I will show how our work has shed light on the physical conditions required to ensure an efficient magnetic field coupling, and present unexpected results regarding the two main agents responsible for the coupling in star-forming cores: dust grains and ionized gas. Following this Ariane thread, I will argue our observational and theoretical findings support a novel scenario where the angular momentum problem for star formation may be actually “solved” not by the formation of large protoplanetary disks but by the combination of 1) lack of organized rotation motions at large envelope radii, 2) the inefficient angular momentum transport due to magnetic braking in the inner envelope (and angular momentum removed through rotating outflows generated by the presence of the magnetic field), and 3) a local origin of the angular momentum incorporated in the star–disk system. Reference review |
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Tuesday December 13 2022 | Recent hiree seminar Réza ANSARI (DAp)
Cosmology with 21cm Intensity Mapping Summary [click here] Intensity Mapping (IM) has been proposed about 15 years ago as an efficient technique to perform cosmological surveys. The 21cm hyperfine transition of neutral hydrogen can indeed be used to map the 3D distribution of matter in the universe, over a wide range of redshifts, from z=0 to z=3 or even z=6, bringing complementary information to the optical surveys.
Since then, few dedicated instruments have been built (CHIME, Tianlai, BINGO) to explore the feasibility of the method; Other more ambitious instruments, such as HIRAX, CHORD or BINGO will be commissioned in the coming years. Intensity mapping surveys are also envisaged for SKA, in addition to the classical HI source surveys. After presenting the principle of 21 intensity mapping, I will briefly discuss its cosmological promises, as well as some of the associated instrumental and scientific challenges. I will then present some of the results of ongoing observations, focusing on Tianlai, and on PAON4. Tianlai is an international project that operates two pathfinder instruments, a cylinder array and a parabolic array, built in Xinjiang, in western China. PAON4 is a small test interferometer, located in Nançay, used to explore some of the technical aspects of compact radio arrays, operating in transit mode. |
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Tuesday April 18 2023 | Judith IRWIN (Queen's University, Canada)
CHANG-ES — Past and Future Summary [click here] CHANG-ES (Continuum Halos in Nearby Galaxies — an EVLA Survey) is a project to observe 35 nearby galaxies that are edge-on to the line of sight to focus on their radio halos and the disk-halo connection. Wide-band VLA observations at L-band (1.5 GHz) and C-band (6.0 GHz) have provided opportunities to study in-band spectral indices, and observations in all four Stokes parameters with Rotation Measure Synthesis has led to a new understanding of the structure of kpc-scale magnetic fields in disk galaxies. This talk will highlight some of the results of the project and look to the future, as newly completed S-band (3.0 GHz) observations have filled in the L-band to C-band gap and led to the widest contiguous frequency coverage yet seen for galaxies. |
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Friday October 20 2023 | Group seminar Tommaso RONCONI (SISSA, Trieste, Italy)
Introducing Galapy: a fast API for modelling galaxy SEDs with Bayesian sampling Summary [click here] Fostered by upcoming data from new generation observational campaigns, we are about to enter a new era for the study of how galaxies form and evolve. The unprecedented quantity of data that will be collected, from distances only marginally grasped up to now, will require analysis tools designed to target the specific physical peculiarities of the observed sources and handle extremely large datasets. One powerful method to investigate the complex astrophysical processes that govern the properties of galaxies is to model their observed spectral energy distribution (SED) at different stages of evolution and times throughout the history of the Universe.
In this talk, I will introduce GalaPy, a new library for modelling and fitting galactic SEDs from the X-ray to the radio band, as well as the evolution of their components and dust attenuation/reradiation. On the physical side, GalaPy incorporates both empirical and physically-motivated star formation histories, state-of-the-art single stellar population synthesis libraries, a two-component dust model for extinction, an age-dependent energy conservation algorithm to compute dust reradiation, and additional sources of stellar continuum such as synchrotron, nebular/free-free emission and X-ray radiation from low and high mass binary stars. On the computational side, GalaPy implements a hybrid approach that combines the high performance of compiled C++ with the user-friendly flexibility of Python, and exploits an object-oriented design via advanced programming techniques. GalaPy generates models on the fly without relying on templates, thus minimising memory consumption. It exploits fully Bayesian parameter space sampling, which allows for the inference of parameter posteriors and thus facilitates the study of the correlations between the free parameters and the other physical quantities that can be derived from modelling. The API and functions of GalaPy are under continuous development, with planned extensions in the near future. I will showcase the project and present the photometric SED fitting tools already available to users. |
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Tuesday December 12 2023 | Stefano ANDREON (INAF-OA Brera, Milan, Italy)
First massive galaxy clusters emerging from the cosmic web at z~2 Summary [click here] In this talk I report upon our results on the intracluster medium (ICM) of two clusters at the time when first clusters start to emerge from the cosmic web, z~2. Results are derived from new, high resolution, deep SZ and X-ray data providing us with the measurement of the two most distant resolved pressure profiles. IDCSJ1426 cluster at z=1.75 has a core whose properties are not far from the final stage, while the remaining part of the cluster is experiencing a sizable gas, heat and entropy transfer. JKCS041 at z=1.80 is caught just after a major merger event as evidenced by its SZ-X-ray peak offset, its low central pressure, and its low Compton-Y parameter compared to its WL mass. Comparison with plausible descendents shows that its ICM will experience major changes at all radii. |
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Tuesday April 23 2024 | Florent MERTENS (Observatoire de Paris)
Unveiling the Physics of the Cosmic Dawn and the Epoch of Reionisation in the SKA era Summary [click here] Direct observation of the Cosmic Dawn and Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. Several experiments such as LOFAR, MWA, HERA, and NenuFAR are underway aiming at statistically detecting the 21-cm signal fluctuations from the EoR and CD, and paving the way for the SKA EoR CD experiment which will be capable of directly image the large-scale neutral hydrogen structures from these distance epochs. In this talk, I will present recent results from the LOFAR-EoR and NenuFAR Cosmic Dawn collaborations. After publishing the deepest upper limit on the signal power-spectra at z~9, which has made it possible to set constraints on the physics of the IGM during the EoR, the LOFAR-EoR team is progressing towards a deeper upper limit on a broader range of redshift. On the Cosmic Dawn front, the NenuFAR CD team recenty published a first upper limit on the signal power-spectra at z~20. These new results will be discussed, as well as the improvements in instrument calibration and foreground mitigation that enabled these achievements. |
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Tuesday November 19 2024 | Postdoc seminar Jose Ignacio "Nacho" ANEZ LOPEZ (LFEMI)
Signatures of magnetic braking in Class 0 protostars ? Exploring the gas kinematics in magnetized models of low-mass star formation Summary [click here] The role of magnetic braking in regulating gravitational collapse and circumstellar disk during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disk sizes and the launching of high-velocity collimated jets, we aim at more direct tests of the magnetic braking in observations. The study of polarized dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process.
In the present work, we have used both non-ideal MHD models and synthetic observations from the radiative transfer of protostellar formation to put constraints on the magnetically-regulated disk formation scenario. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetized model shows a higher angular momentum redistribution above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O(2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetized model similar to the characteristics found in observational work. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from radial motions. |
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Tuesday April 27 2021 | Frédéric BOURNAUD (Departement d’Astrophysique, IRFU, CEA-Saclay et Université Paris-Saclay)
Cosmological simulations of galaxy formation: toward a robust accounting of galaxy populations and star formation across the Universe? Summary [click here] Cosmological simulations of galaxy formation are reaching a high level of accuracy and can finely reproduce some of the main properties of galaxy populations: stellar masses, angular momentum, colors, etc. However, most galaxy formation simulations still fail to account for the detailed structure of galaxies and their global star formation history. Using high-resolution, idealized simulations of galactic dynamics and star formation, I will show that these disagreements are not cosmetic details but point toward a fundamental tension between observations and galaxy formation models. Historically, galaxy formation models predicted galaxies with unrealistically large stellar masses: in modern cosmological simulations, this issue is generally solved though energetic feedback from young stars and supermassive black holes. I will nevertheless show that feedback, as implemented in such simulations, is generally excessive, leading to the early and unrealistic exhaustion of interstellar gas reservoirs. Comparisons to idealized simulations and observations of galactic winds support the conclusion that energetic stellar and black hole feedback cannot be entirely responsible for the regulation of star formation and galaxy growth. Other physical processes likely emerge from sub-galactic scales in the interstellar medium, such as subtle coupling between galactic dynamics and star formation through instabilities and turbulence. Nevertheless, some cosmological simulations can now successfully describe the re-distribution of baryons from galaxies to the intergalactic medium, and I will show that these simulations are a crucial tool not just for galaxy formation but also for modern cosmological surveys. I will finally review how simulations of galaxy formation, evolution and star formation could gain strength in the years to come, in relation with the arrival of exascale supercomputers. |
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Tuesday February 1 2022 | Pascal TREMBLIN (Maison de la Simulation)
Non-ideal self-gravity and cosmology: the importance of correlations in the dynamics of the large-scale structures of the Universe
Summary [click here] Inspired by the statistical mechanics of an ensemble of interacting particles (BBGKY hierarchy), we propose to account for small-scale inhomogeneities in self-gravitating astrophysical fluids by deriving a non-ideal Virial theorem and non-ideal Navier-Stokes equations using a decomposition of the gravitational force into a near- and far-field component. These equations involve the pair radial distribution function (similar to the two-point correlation function), similarly to the interaction energy and equation of state in liquids. Small-scale correlations lead to a non-ideal amplification of the gravitational interaction energy, whose omission leads to a missing mass problem, e.g., in galaxies and galaxy clusters. We also propose an extension of the Friedmann equations in the non-ideal regime. We estimate the non-ideal amplification factor of the gravitational interaction energy of the baryons to lie between 5 and 20, potentially explaining the observed value of the Hubble parameter. Within this framework, the acceleration of the expansion emerges naturally because of the increasing number of sub-structures induced by gravitational collapse, which increases their contribution to the total gravitational energy. A simple estimate predicts a non-ideal deceleration parameter qni~-1; this is potentially the first determination of the observed value based on an intuitively physical argument. We suggest that correlations and gravitational interactions could produce a transition to a viscous regime that can lead to flat rotation curves. This transition could also explain the dichotomy between (Keplerian) LSB elliptical galaxy and (non-Keplerian) spiral galaxy rotation profiles. Overall, our results demonstrate that non-ideal effects induced by inhomogeneities must be taken into account in order to properly determine the gravitational dynamics of galaxies and the larger scale universe. |
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Tuesday March 22 2022 | Benjamin WEHMEYER (CSFK, Budapest)
Galactic Chemical Evolution of rapid neutron capture process elements using special, rare classes of supernovae, and of short lived radioisotopes Summary [click here] The origin of the heaviest elements is still a matter of debate. For the rapid neutron capture process (r-process), multiple sites have been proposed, e.g., neutron star mergers and (sub-classes) of supernovae (e.g., magnetorotationally driven supernovae). R-process elements have been measured in a large fraction of metal-poor stars. Galactic archaeology studies show that the r-process abundances among these stars vary by over 2 orders of magnitude. On the other hand, abundances in stars with solar-like metallicity do not differ greatly. This leads to two major open questions:
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Tuesday April 5 2022 | Barbara OLMI (INAF, Italy)
Modeling Pulsar Wind Nebulae through their evolutionary phases Summary [click here] Pulsar wind nebulae are fascinating systems, powered by the central rotating compact star, emanating a wind in the form of a relativistic, magnetized, and cold plasma that fills the nebula. They are visible as bright non-thermal sources in a very broad range of energies, from radio to gamma-rays. Observed morphologies vary with the evolutionary phase, with middle-aged and old systems strongly affected by the interaction with the ambient medium. Modeling of these sources requires some carefulness when going through the various phases, with a comprehensive description still lacking.
Pulsar wind nebulae had been for a long time thought to contribute substantially to the positron excess in the CR spectrum at Earth -- potentially being the primary sources. In the last years, numerous evidence for efficient particle leakage by aged nebulae had been collected, showing up as quasi-monochromatic misaligned jets at X-rays in some cases, or in the form of extended TeV halos in others, reanimating somehow the interest in this class of objects. Here I will review our present knowledge of pulsar wind nebulae models through their different ages. |
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Tuesday June 21 2022 | Recent hiree seminar Sandrine CODIS (DAp)
Modeling the birth and growth of the cosmic web Summary [click here] Starting from the largest scales, I will first describe how the cosmic web is woven across cosmic time into a gigantic bubble-like tapestry made of nodes, filaments, walls and voids. A particular emphasize will be put on the geometry and connectivity of this cosmic foam. Recent theoretical works aiming to precisely model the Universe on those mildly non-linear scales will be presented. In particular, I will identify a regime where large-deviation theory can be successfully implemented to predict the so-called count-in-cells statistics and describe promising cosmological applications for future galaxy surveys. The second part of the talk will focus on the birth and evolution of haloes and galaxies within these large cosmic highways. The highly anisotropic galactic environment set by the cosmic web will be shown to play a significant role in shaping them, an effect inducing large-scale galaxy alignments that are difficult to model but represent an important contamination for weak lensing experiments. |
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Tuesday September 6 2022 | Group seminar Nicolas SCEPI (CU Boulder)
Formation, Evolution and Radiation of Magnetized Accretion Disks around Black Holes Summary [click here] The impact of magnetic fields on the evolution and on the observational signatures of accretion disks is very uncertain. This uncertainty is mainly due to a lack of observational constraints on the magnetic field geometry or strength in accretion disks. However, even from a theoretical point of view our understanding of magnetized disks remains relatively poor. Indeed, analytic models of magnetized disks often need inputs from numerical simulations and numerical simulations of magnetized disks are difficult to perform and/or interpret. Because of this lack of magnetized disk models, standard disk models often reduce the magnetic field to a source of turbulence; turbulence through which the accretion can happen. While this simplification may hold for weakly magnetized disks, a large number of numerical simulations have shown that the role of a strong magnetic field goes far beyond producing turbulence. In particular, a strong magnetic field can produce powerful outflows, induce accretion through vertically elevated layers or non-axisymmetric structures, modify the time scales of accretion, enhance dissipation of gravitational energy in the disk and accelerate particles to very high energies. All of these effects dramatically affect the evolution and observational signature of accretion disks and open up new and exciting avenues to resolve outstanding problems of the standard accretion disk theory. In this talk, I will present an overview of my recent results on how strongly magnetized disks form, evolve and radiate. I will show in particular how strongly magnetized disks could explain events of very strong variability in AGNs, the flaring behavior of the Galactic center and the hardest emission in X-ray binaries. |
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Thursday October 13 2022 | Group seminar Jordan PHILIDET (Max Planck Institute for Solar System Research, Göttingen)
Impact of turbulent convection on stellar oscillations Summary [click here] The study of the global oscillations of stars, which is at the heart of asteroseismology, has enabled us to make significant progress in our
understanding of stellar physics. In Sun-like stars, these oscillations are affected by the turbulent motions entailed by the convective instability in the envelope. In addition to modifying the resonant frequencies of the modes, turbulent convection is also responsible for their excitation, as well as part of their damping. As such, solar-like oscillations gives us access to crucial information about stellar convection, which constitutes one of the main obstacles towards better stellar modelling. However, in order to exploit the wealth of asteroseismic data at our disposal to better constrain the properties of stellar convection, it is necessary to theoretically model the relation between these properties and the asteroseismic observables (namely the amplitude of the modes, their lifetime, and the amount by which convection shifts their frequencies, referred to as surface effects).
In this talk, I will first focus on solar-like acoustic modes. Traditional approaches to study the effect of convection thereon are either based on parametric empirical formulations, or else on 3D simulations. These approaches show unavoidable limitations, among which the impossibility to realistically describe the full turbulent cascade, and especially the turbulent dissipation of kinetic energy. Here, I will present a new alternative theoretical framework designed to circumvent these limitations, based on Lagrangian stochastic models. I will demonstrate how, under reasonable assumptions, this sort of formalism can lead to simultaneous theoretical estimates for the amplitude, lifetime and surface effect of the acoustic modes, directly as a function of the turbulent fluctuations caused by convection, thus allowing to constrain turbulent convection models through the direct comparison of these estimates with observed mode properties. The second part of this talk will focus on another kind of oscillations, namely the inertial modes recently observed on the surface of the Sun. These modes propagate under the action of rotation, through the Coriolis acceleration, and, for the most part, are predicted to be stable, meaning that they are likely also excited by turbulent convection, just like acoustic modes. In order to test that hypothesis, I will present a theoretical formalism where the turbulent velocity fluctuations provide the mechanical work necessary to excite the modes, which are described by means of a 2D linear wave equation, under the β-plane approximation. Based on the general agreement between the predicted and observed inertial mode amplitudes, I will show that the (linearly stable) solar inertial modes are indeed excited by turbulent convection. This formalism also shows that the power in high azimuthal order spectra is not easily separable into individual modes, thus complicating the interpretation of the observations. |
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Friday November 18 2022 | Group seminar Camille DIEZ (University of Tübingen)
Using Vela X-1 to understand accretion and wind structure in neutron star High-Mass X-ray Binaries (HMXBs) Summary [click here] The spectral and timing behaviour of HMXBs offers a unique opportunity for the investigation of accretion onto compact objects and of wind structure in massive stars. The bright and persistent neutron star HMXB Vela X-1 is one of the key systems for such studies with both current and future instruments. It has a complex clumpy stellar wind, prominent cyclotron resonant scattering features (CRSFs) and strong flares. Understanding the variability of the systems on both short time scales of a few hundreds seconds and along its 9d orbit with current instruments enables us to make predictions for future observations with XRISM and Athena and to devise the best observational strategy for Vela X-1. Here, we analyse two new observations taken with NuSTAR and XMM-Newton at orbital phases ~0.5 and ~0.75 and follow the evolution of spectral parameters down to the pulse period (~300s) time-scale. The flux-dependency we observe in the spectral shape implies a change in the properties of the Comptonising plasma and the observed drop of the CRSF energy following a strong flare may indicate a change in the accretion geometry. The strong variability of absorption is due to the presence of a large-scale wind structure, such as accretion- and photoionisation wakes, combined with the variable line of sight as the neutron star moves along the orbit. In particular, we, for the first time, are able to trace the onset of the wakes with high time resolution and compare to predictions from simulations. |
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Tuesday January 17 2023 | Ludovic PETITDEMANGE (LRA/LERMA)
Dynamo action and angular momentum transport in simulated stellar radiative zones Summary [click here] The evolution of a star is influenced by its internal rotation dynamics through transport and mixing mechanisms, which are poorly understood. Magnetic fields can play a role in transporting angular momentum and chemical elements, but the origin of magnetism in radiative stellar layers is unclear. Using global numerical simulations, we identify a subcritical transition to turbulence due to the generation of a magnetic dynamo. Our results have many of the properties of the theoretically-proposed Tayler-Spruit dynamo mechanism, which strongly enhances transport of angular momentum in radiative zones. It generates deep toroidal fields that are screened by the stellar outer layers. This mechanism could produce strong magnetic fields inside radiative stars, without an observable field on their surface. Magnetic fields generated by dynamo action appear as a process to trigger turbulence in stellar interiors. Depending on the parameters or initial conditions, we report different dynamo branches that could explain stellar magnetism and the rotation profiles observed for stars having a thick radiative envelope. |
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Tuesday March 14 2023 | Special seminar Marie GUÉGUEN (Université de Rennes)
Cosmology: a tension within code comparisons Summary [click here] Code comparisons in cosmology are often performed with the underlying objective of identifying predictions upon which different codes converge that can be interpreted as robust, free of artifacts, predictions. Such an objective assumes that code comparisons can be constructed in such a way that the two notions of convergence and of robustness collapse. But, in order to achieve such a goal, code comparisons of structure formation have to meet an important epistemic challenge: that of constructing their codes ensemble on the basis of codes that are as independent as possible, but also comparable. In this talk, I show that enforcing the latter often amounts to multiplying common idealizations that hinder the achievement of the former, and thus leads to code comparisons that include many unscrutinized sources of artefacts possibly similarly distorting the predictions of the model. As a result, I argue that in context of high uncertainties where the domain of verification and validation of simulations has shrunk to code comparisons, code comparisons are better and actually very efficient as exploratory tools, both for getting insights into the physics implemented and for breaking the epistemic opacity of numerical simulations. |
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Tuesday April 4 2023 | Andrei IGOSHEV (University of Leeds)
Magneto-thermal evolution of neutron stars Summary [click here] We perform first three-dimensional simulations of the magneto-thermal evolution using a spectral MHD code for crust confined magnetic field configurations. Our results show that presence of strong toroidal magnetic field in magnetars is necessary to explain their quiescent thermal emission, in particular a formation of a single hot spot. Using our thermal maps we are able to explain light curves of 10 out of 19 magnetars in quiescence. In the case of the central compact objects, we test the configuration of magnetic field formed as a result of stochastic dynamo. Such a magnetic field consists of multiple randomly orientated loops of magnetic field. Surface thermal map is becoming patchy and includes multiple hot and cold regions which are always observed simultaneously. The global dipolar field slowly formed as a result of the Hall and Ohmic evolution. In our simulations we see 5-10% pulsed fraction and difference of two times in temperature between hot and cold regions typical for observations of the central compact objects. We also study off-centred dipole configurations and found that they decay over time. |
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Tuesday April 11 2023 | Anne VERHAMME (Université de Genève)
Searching for the sources of Cosmic Reionisation Summary [click here] Over the past 5 years, we have made tremendous progress on both direct detections of the escape of ionizing radiation from galaxies, over a broad range of redshifts (and instruments), and the tests and validations of indirect probes of the escape of ionizing radiation from galaxies, both from observations and simulations. I will review these recent achievements, and describe the next steps to understand the nature of the sources of reionisation. |
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Tuesday June 13 2023 | Postdoc seminar Ugo LEBREUILLY (LMPA)
Gas and dust evolution during the formation of protoplanetary disks Summary [click here] Protoplanetary disks are the consequence of angular momentum conservation during the protostellar collapse. Their formation is a complex process which includes numerous physical effects (non-ideal MHD, stellar feedback, gas and dust interactions, turbulence…). In this seminar, I will
present our recent works to better understand the formation of these disks. In the first part of the talk, I will focus on their gas content. I will show how modelling simultaneously the large scales of star forming regions and the small scales of protoplanetary disks allows us to constrain the statistical properties (mass, radius, temperature…) of these disks. The second part of the talk will be dedicated to the study of dust evolution. In particular, I will stress the consequences of this process not only for the formation of disks, but also for the formation of planets. |
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Tuesday June 27 2023 | Julien AUBERT (IPGP)
Variations géomagnétiques rapides: un nouveau message émis par le noyau de la Terre Summary [click here] Le signal géomagnétique est une source riche d’informations sur la structure, la dynamique interne et l’histoire de notre planète. La production du champ magnétique Terrestre par effet dynamo dans le noyau externe implique une large disparité d’échelle spatiales et plus spécifiquement temporelles, s’étalant sur une gamme allant de l’année au milliard d’années. Depuis la mise en place d’observatoires magnétiques à la surface émergée de la Terre, l’attention s’est portée sur l’explication des variations à l’échelle du siècle, qui sont liées aux mouvements de convection dans le noyau. Depuis une vingtaine d’années, une couverture satellitaire globale et continue a cependant mis en évidence des variations de l’année à la dizaine d’années, dont l’origine est débattue. Ces nouvelles données ouvrent une fenêtre sur des phénomènes magnétohydrodynamiques rapides, de nature ondulatoire, en interaction avec la convection lente dans le noyau. Dans cet exposé, je présenterai les défis posés par la simulation conjointe des deux phénomènes ainsi que les avancées récentes et applications géophysiques potentielles. |
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Tuesday September 26 2023 | Postdoc seminar Adam FINLEY (LDE3)
Studying the whole Sun: from small-scale heating to large-scale dynamics Summary [click here] The Sun’s large-scale magnetic field undergoes periodic reversals due to dynamo-action in the solar interior, through which the Sun’s magnetic field regenerates. The emergence of new magnetic field at the solar surface, after buoyantly rising through the convection zone, is clearly
visible due to the formation of dark spots (sunspots). However, current models of the solar dynamo are unable to self-consistently capture the formation of sunspots, due to the range of pressure scale heights needed to include the photosphere. Thus, dynamo models remain disconnected from sunspot observations. The cyclic evolution of the Sun’s magnetic field also has a clear impact on the structure of the solar atmosphere and outflowing wind above. Similarly, linking the evolution of different scales, from the buffeting of convective motions in the photosphere to the dissipation of Alfven waves in the solar wind. Modern models of the Sun, therefore, require the combination of expertise from a range of interconnected subject areas. In this talk, I will highlight some of the recent work from the WholeSun ERC Synergy grant (https://wholesun.eu), which
brings together expertise from five different host institutions across Europe. These works range from assessing the observational signatures of toroidal flux generation, to modelling small-scale energy injection at the base of the solar wind, and finally, estimating the large-scale variation in coronal structure and rotation during the solar cycle. |
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Tuesday October 17 2023 | Postdoc seminar Sudip CHAKRABORTY (LEPCHE)
Unraveling the magnetic origin of dense non-equatorial ultrafast outflows in black hole X-ray binaries Summary [click here] Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era. |
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Tuesday November 21 2023 | Recent hiree seminar Camila CORREA (LCEG)
Journey into the Unknown: Dark Matter, Observations of Galaxies, and the Path Forward Summary [click here] The nature of dark matter remains as one of the big unknowns of our time. The general expectation for the coming years is that Euclid, DESI, Rubin LSST, among others, will facilitate the production of wide-field galaxy surveys with exceptionally precise measurements, which will be crucial for unraveling the mysteries surrounding dark matter. We eagerly anticipate that the new observations will reveal deviations from the canonical cold collisionless dark matter paradigm, offering insights into its true nature. However, the key question lingers: will it really happen? What is missing now, that Euclid and others state-of-the-art facilities will change? In this seminar, I will strive to address these questions and review the latest status on dark matter searchers, in order to give you an impression of where we are in the search for dark matter and where we are going.
Throughout the seminar, I will also provide an overview of my work on the nature of dark matter and delve into the pivotal role that cosmological simulations of galaxy formation play in this quest. In the interpretation of data from both current and upcoming state-of-the-art observatories, cosmological simulations have emerged as indispensable tools. Simulations have convinced us of the success of LCDM over large scales, and have given us the capability of breaking the degeneracy driven by baryonic physics and dark matter models. However, can we really trust the outcomes of simulations? with their limited resolution and ad-hoc subgrid prescriptions for galaxy evolution. Maintaining a critical perspective on the observational data we work with and the simulated data we generate daily is crucial for advancing in this field. During the seminar, I will provide updates on the current challenges faced by cosmological simulations and highlight the progress they have achieved. I plan to be as efficient as possible to ensure time for a concluding session that hopefully leaves you with food for thought—an engaging discussion about the future. What steps lie ahead in the development of cosmological simulations? What about on the nature of dark matter? What synergies are needed to be forged between theoretical advancements and observational endeavours? And, importantly, what role will DAp play in this unfolding narrative? Let’s chat more on Tuesday 21 Nov. at 10 am. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Tuesday February 6 2024 | Stefanie WALCH-GASSNER (University of Köln)
The impact of stellar feedback on different scales and galactic environments Summary [click here] Stellar feedback refers to the processes by which massive stars release energy, radiation and material into their surroundings, influencing the structure and evolution of the galaxies in which they reside. Understanding the impact of stellar feedback on different galactic environments is crucial for developing a comprehensive picture of galaxy formation and evolution. In this context, different galactic environments refer to regions within a galaxy that differ in their physical conditions, such as the average gas density, temperature, or metallicity.
We study the respective impact of stellar winds, ionizing radiation, and supernovae in modern simulations of the multi-phase interstellar medium in parts of galaxies within the SILCC project, which I will present in this talk. From these galactic scale simulations we find that ionizing radiation is the most important factor in regulating the star formation rate, while supernova over-pressure the gas substantially, thus driving a galactic outflow. |
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Tuesday February 13 2024 | Jonathan TAN (Chalmers & University of Virginia)
A Light in the Dark - Massive Star Birth Through Cosmic Time Summary [click here] Massive stars are important thoughout the universe, but their formation remains poorly understood. I review current understanding of how massive stars and star clusters form in our Galaxy, including models for how star formation is triggered in giant molecular clouds and tests of how individual massive stars form from smaller scale clumps and cores. Finally, I discuss how massive star formation may have been different in the very early universe and how the first stars may have seeded the supermassive black holes powering active galactic nuclei. |
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Tuesday March 12 2024 | Postdoc seminar Arturo NUNEZ (LMPA)
The impact of baryonic physics in galaxy formation: Multi-scale approach to galaxy simulations, from the initial mass function to galaxies in a cosmological environment Summary [click here] The interplay of baryonic physics (star formation and feedback processes) in shaping galaxies and their host halos is a multiscale and multidisciplinary problem. Numerical simulations are key to understanding these processes as they cover phenomena that take place at a wide range of scales, some too small for most astrophysical observations but yet able to impact the shape and evolution of galaxies as a whole. However, no single simulation can address every aspect of this complex issue. A comprehensive approach is essential to integrate learnings from various types of simulations and compare them with observations. This includes understanding star formation histories in cosmological simulations, examining interstellar medium dynamics in comparison to simulations of individual galaxies, and exploring the non-universality in the stellar initial mass function in specific molecular cloud simulations. My goal is to discuss how simulations across different scales, together with detailed multiscale observations, collectively contribute to approaching the complexities of galaxy formation and evolution. All this while discussing current shortcomings and successes of high-resolution numerical simulations of galaxies and galactic environments.
WARNING: incomplete video recording. |
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Tuesday September 10 2024 | Postdoc seminar Maxime LOMBART (LFEMI)
How to treat dust coagulation/fragmentation in 3D hydrodynamic simulations ? Summary [click here] Particles coagulation and fragmentation are ubiquitous (raindrop formation, air pollution, combustion, polymerization, astrophysics) and mathematically described by the Smoluchowski coagulation and the fragmentation equations. Several processes such as gas/particles dynamics, chemical reaction and radiative transfer depends on the evolution of the particle size distribution governed by coagulation/fragmentation. Tracking the size evolution of particles in 3D simulation is key for understanding, for instance, cloud formation and planet formation. Therefore, these equations must be accurately solved while preserving computational costs, which is a tremendous numerical challenge. However, current algorithms for solving coagulation/fragmentation suffer from an over-diffusion in the conditions of 3D simulations. To tackle this challenge, we developed a discontinuous Galerkin scheme to efficiently solve the conservative form of the coagulation/fragmentation equation. In particular, we aim to perform the first 3D simulations of dusty protoplanetary discs and protostellar collapse that include realistic coagulation/fragmentation. |
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Tuesday September 17 2024 | Postdoc seminar Arghyadeep PAUL (LDE3)
Star-Planet Magnetic Interactions: From Solar System to Exoplanets Summary [click here] The interaction between stellar winds and planetary magnetospheres has been a focus of research for decades. Within the heliospheric context, star-planet interactions similar to that between the Sun and the Earth reshapes the planet’s local magnetic environment leading to the formation of magnetospheres. Magnetic reconnection within these magnetospheres gives rise to helical magnetic flux ropes known as flux transfer events (FTEs). We developed a two-way coupled magnetosphere-ionosphere model to investigate the impact of FTEs on the planet’s ionosphere. For a specific case of an Earth like planet, the field aligned currents generated by these FTEs closely resemble observations of discrete dayside auroral arcs, suggesting FTEs to be a probable cause. In the context of exoplanetary systems, star-planet interactions vary depending on the planet's orbital location. For close-in orbits located in the sub-Alfvénic stellar wind, the Poynting flux generated by star-planet interactions can propagate toward the star, giving rise to stellar chromospheric hotspots. Our current work characterises and quantifies the efficiency of this energy transfer
between the planet and the star, revealing that a significant portion of the energy never reaches the star and is reflected by the stellar transition region back toward the planet. The firm detection and characterisation of such magnetic interactions from observations of chromospheric hotspots would also lead to constraints on the amplitude of the magnetic field of exoplanets, to which we are blind so far. Future research, utilising the previously developed magnetosphere-ionosphere model, will also explore how the presence or absence of a planetary ionosphere influences the Poynting flux generated by the planet. |
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Tuesday November 19 2024 | Postdoc seminar Jose Ignacio "Nacho" ANEZ LOPEZ (LFEMI)
Signatures of magnetic braking in Class 0 protostars ? Exploring the gas kinematics in magnetized models of low-mass star formation Summary [click here] The role of magnetic braking in regulating gravitational collapse and circumstellar disk during the main accretion phase, is an open question. While only indirect evidence was found from observational work, such as compact disk sizes and the launching of high-velocity collimated jets, we aim at more direct tests of the magnetic braking in observations. The study of polarized dust emission as a tracer of the magnetic field and molecular line emission as a tracer of gas kinematics in young protostars can provide valuable information for understanding how the presence of the magnetic field affects the accretion process.
In the present work, we have used both non-ideal MHD models and synthetic observations from the radiative transfer of protostellar formation to put constraints on the magnetically-regulated disk formation scenario. We use our model synthetic observations to identify possible direct signatures of the magnetic braking from the maps of the molecular gas emission. By comparing the specific angular momentum of two similar models that differ in magnetic flux, we see that the more magnetized model shows a higher angular momentum redistribution above 1000 au. In addition, we have tested the methods typically used to infer the specific angular momentum from an observational point of view. We have found possible observational evidence of magnetic braking in the kinematics of the C18O(2-1) molecule, such as a flattening of the radial profile of specific angular momentum for radii smaller than 1000 au for the more magnetized model similar to the characteristics found in observational work. On the other hand, in this study we show that the maximum velocity computed in the equatorial plane, which is traditionally used as an approximation of the rotational velocity, may overestimate the rotational velocity, probably due to contamination from radial motions. |
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Tuesday April 19 2022 | Sacha BRUN et Olivier LIMOUSIN (DAp)
Solar Orbiter: the heliospheric explorer Summary [click here] Solar Orbiter - ESA M1 Mission - has entered its scientific phase at the end of November 2021, after a cruise phase of more than 18 months (and 2.2 Billion km) and the commissioning of the 10 instruments on board. CEA/IRFU played a key role in this mission, by providing the focal plane detector array of the X-ray telescope, STIX, based on in-house Caliste technology. In the meantime, solar physicists have been busy preparing the pipelines needed to handle the data sent by the instrument, and developing high performance numerical simulations of the Sun.
In this 2-voice seminar, we will relate the first 26 months of activities of the mission, covering both STIX calibration, its first light, the first solar flare detected, as well as the development of associated numerical simulations of the Sun and optimal scientific processing of instrumental data sent as the spacecraft gets closer and closer to the Sun (last perihelion was on 26 March 2022 at 0.32 AU), while in the meantime our star is increasing in intensity and in activity, with solar magnetic cycle 25 now well on its way. |
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Tuesday June 14 2022 | Diego GÖTZ et Aline MEURIS (DAp)
The MXT telescope on board SVOM: a new tool for time-domain and multi-messenger astrophysics Summary [click here] The Sino-French SVOM (Space based astronomical Variable Object Monitor) mission is ready for launch in 2023. This mission is dedicated to the study of Gamma-Ray Bursts and other transient and variable sources of the high-energy sky. On board SVOM there will be four instruments, ECLAIRs and GRM, with large field of views, operating in the hard X- and gamma-ray domain, and two narrow field instruments: the Visible Telescope (VT) and the Microchannel X-ray Telescope (MXT). The MXT is a novel kind of compact and light instrument based of the « Lobster Eye » optical concept, coupled to a low noise state-of-the art X-ray camera, the latter being designed and manufactured at CEA Irfu. For a total mass of 42 kg and a total power of 60 W, this instrument is composed of an optics system, a telescope tube in carbon fiber, a radiator, a camera and a data processing unit. The 9 kg camera consists of a focal plane assembly with a detector assembly and thermoelectrical coolers, a front-end electronics assembly, a calibration wheel assembly and a support structure assembly. We will first review the SVOM scientific objectifs and how MXT will contribute to reach them. Then we will present the MXT design in more detail, focussing on the CEA contribution, and finally we will present the results of the calibration campaign performed in 2021 before the delivery of the telescope to Cnes. |
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Tuesday November 8 2022 | Philippe Laurent, Aymeric Sauvageon, Jérôme Rodriguez (DAp)
INTEGRAL : 20 ans dans l'espace pour une aventure de plus de 35 ans Summary [click here] Le laboratoire INTErnational d'Astrophysique des Rayons Gamma (INTEGRAL) a été lancé le 17 octobre 2002 de Baikonour (Kasakstan). Depuis lors il est resté sur son orbite elliptique de haute excentricité (environ 3 jours) effectuant 2568 révolutions (au 8 novembre 2022) autour de la Terre pour environ 530 Ms (méga secondes) d'observations scientifiques. Cette mission de taille moyenne transporte deux instruments principaux opérant dans le domaine spectral des rayons X durs et des rayons gamma mous (20keV-10 MeV), dont l'optique est basée sur le concept de masques codés : l'un est dédié à l'analyse spectrale fine avec des capacités d'imagerie modérées (SPI), l'autre est dédié à l'imagerie fine (ou presque) avec des capacités spectrales modérées (IBIS).
Ces 20 ans dans l'espace sont le résultat d'au moins 15 ans de développement du projet avant qu'INTEGRAL puisse dévoiler le mystère du ciel à haute énergie. Le Dap a été profondément impliqué dans cette aventure de longue haleine, depuis le tout début du développement du concept de la mission (grande implication dans les deux instruments, ISGRI et SPI, participation au centre de données, développement de l'analyse s/w, suivi de la caméra) jusqu'à l'analyse actuelle des données en temps réel et des archives, avec un grand nombre d'articles publiés dans tous les domaines permis par les capacités instrumentales et même au-delà. Nous allons, dans ce séminaire, résumer ces plus de 35 ans en présentant divers aspects du projet/de la mission, en nous concentrant particulièrement sur ceux où l'implication du Dap a été cruciale. Nous commencerons par une vue d'ensemble de l'histoire de la mission, des principales caractéristiques instrumentales et des objectifs scientifiques au lancement, puis nous nous concentrerons sur la caméra ISGRI, la couche de détection 20-250 keV du télescope IBIS, qui est suivie au Dap tous les jours. ISGRI a été développée à l'IRFU et a obtenu la plupart des résultats d'INTEGRAL. Nous conclurons par un aperçu rapide et évidemment biaisé de quelques résultats scientifiques obtenus au cours de ces 20 années. Le séminaire sera présenté en Français avec des diapositives en Anglais par Philippe Laurent, Aymeric Sauvageon et Jérôme Rodriguez, bien humblement au nom d'un grand nombre de collègues anciens et actuels. ―――――――――――――――――――――――――――――――――――――――――― ENGLISH VERSION INTEGRAL : 20 years in space and for a 35+ years adventure The INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) has been launch on October 17th, 2002 from Baikonour (Kasakstan). Since then it has remained on its high-eccentricity elliptical orbit (about 3 days) performing 2568 revolutions (as of Nov. 8th, 2022) around the Earth for around 530 megaseconds of scientific observations. This medium sized mission caries two main instruments operating in the spectral domain of hard X-rays/Soft Gamma-rays (20keV-10 MeV), whose optics is based on the concept of coded masks: one is dedicated to fine spectral analysis with moderate imaging capabilities (SPI) the other is dedicated to fine(-ish) imaging with moderate spectral capabilities (IBIS). These 20 years in space are the results of at least 15 years of project development before INTEGRAL could unveil the mystery of the high-energy sky. The DAp has been deeply involved in this long-term adventure from the very beginning of the mission concept development (large involvement in both instruments, ISGRI and SPI, participation to the data centre, development of the s/w analysis, monitoring of the camera) to the current analysis of real time and archival data, with a large number of published papers in all fields allowed by the instrumental capabilities and even beyond. We will, in this seminary, summarised these 35 years+ by presenting various aspects of the project/mission, focusing especially on those where the involvement of the Dap has been crucial. This will start with an overall overview of the mission history, the main instrumental characteristics and scientific goals at launch, followed by a focus on the ISGRI camera, the 20-250 keV detector layer of the IBIS telescope, which is followed-up at DAp every days. ISGRI has been developed at IRFU and obtained most of the INTEGRAL results. We will conclude with a quick and obviously biased overview of some scientific results obtained over these 20 years. The talks will be given by Philippe Laurent, Aymeric Sauvageon, and Jérôme Rodriguez humbly on behalf of many, many former and current colleagues and presented in French with slides in English. |
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Tuesday November 22 2022 | Recent hiree seminar Antonio GARCIA MUÑOZ (DAp)
Exoplanet atmospheres: An anticipated revolution Summary [click here] The launch of the JWST less than a year ago is expected to set a turning point in exoplanet science, which is progressively transitioning from detection and population statistics to in-detail characterization of the exoplanets’ atmospheres. In this talk, I will present an admittedly biased
perspective of what this may represent for our understanding of exoplanets, and how ongoing theoretical work and future telescopes may build upon JWST’s legacy. |
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Tuesday June 6 2023 | Postdoc seminar Carlos GÓMEZ GUIJARRO (LCEG)
Understanding the role of compact star formation in galaxy evolution with ALMA and JWST Summary [click here] Massive elliptical galaxies in the local universe appear to have their high-redshift analogs in the form of extremely compact quiescent galaxies. Therefore, it seems that compact star formation appears to play a pivotal role in the evolutionary pathways of massive galaxies across cosmic history. However, it remains to be understood what this role is in the broader picture set by the main sequence and the scaling relations in galaxy evolution. From an ALMA survey at 1.1mm, we reveal that compact star formation appears to be the norm in massive star-forming galaxies, and sizes as extended as typical star-forming stellar disks are rare. A population of galaxies with modest star formation rates, but which exhibit extremely compact star formation with starburst-like depletion timescales unveils. Compact star formation appears as a physical driver of depletion timescales, gas fractions, and dust temperatures. Gas and star formation compression seems to be a mechanism that allows to hold their star formation rate even when their gas fractions are low and they are presumably on the way to quiescence. Another population of galaxies missed in the deep optical surveys but bright at far-IR/mm wavelengths unveils thanks to recent JWST observations. We present a study investigating the drivers of dust attenuation in massive galaxies in the JWST-era, showing how the stellar mass and morphology plays an important role, with evidence for more compact stellar profiles resulting in the obscuration of galaxies. |
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Thursday June 29 2023 | Julien DE WIT (Department of Earth, Atmospheric and Planetary Science, MIT, USA)
Robustly Characterizing Exoplanets in a New Era of High-Precision Transit Measurements Summary [click here] With a new-generation of great observatories coming online this decade, unprecedented insights into exoplanets will soon be within reach. Observatories such as the James Webb Space Telescope (JWST) notably enable the study of atmospheres around terrestrial exoplanets and can reveal tri-dimensional structures in the atmospheres of their larger counterparts. Robustly leveraging new observations to reach such achievements will however require extra care as the models currently used may not be up to par with their precision.
During this presentation, I will introduce work done by MIT’s Disruptive Planets group and collaborators towards supporting the robust in-depth characterization of exoplanets. I will specifically discuss how not accounting for the true shape of a planet can lead to a misinterpretation of its interior properties as well as atmospheric structure; how the current state of our understanding of light-matter interactions can similarly affect our interpretation of planetary spectra and thus inferences regarding their atmospheric properties; and how the current state of emission spectrum models for stars may even prevent from disentangling between the contribution of a planet and its host star, to start with. I will also present possible ways to address these challenges. I will end with a step-by-step roadmap to the robust characterization of temperate terrestrial planets with JWST, which includes habitability assessment. |
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Tuesday September 12 2023 | Alexander MOISEEV (NASA/GSFC and University of Maryland)
New Mission Concept: Compton Telescope with Coded Aperture Mask, and its Science Perspectives Summary [click here] The Galactic Explorer with a Coded Aperture Mask Compton Telescope (GECCO) is a novel Explorer-class concept for a next-generation telescope covering the poorly explored hard X-ray and soft gamma-ray energies. This concept builds upon the heritage of past and current missions, improving sensitivity and, very importantly, angular resolution. GECCO uses the combined Coded Aperture Mask and Compton telescope techniques to employ the benefits of both: superior angular resolution provided by the deployable Coded Aperture Mask, and good background rejection and wide field-of-view (FoV) provided by the Compton telescope. It is being developed at NASA/GSFC in collaboration with other US and foreign institutions. GECCO observations will extend arcminute angular resolution to high-energy images of the Galactic plane, combining the
spectral capabilities of INTEGRAL/IBIS and the x-ray imaging of NuSTAR and eROSITA, and will make a bridge to the Fermi-LAT observations, enabling a broad potential for discoveries in the MeV γ-ray sky.
With the unprecedented angular resolution of the coded mask telescope combined with the sensitive, wide FoV Compton telescope, GECCO will focus on two main science objectives:
The GECCO design is based on the novel CZT Imaging calorimeter, which serves as a standalone Compton telescope and as a focal plane detector for the Coded Mask. It also is a powerful tool to measure the γ-radiation polarization. GECCO’s octagon-shaped active shield also serves as a powerful all-sky detector of gamma-ray bursts, prompting the instrument to slew towards the burst direction and localize it with the Coded Aperture’s arcminute accuracy. GECCO will operate in the 100 keV - 10 MeV energy range, with energy resolution of ~ 1% from 0.5 - 5 MeV. The Coded Aperture Mask provides the angular resolution of ∼1 arcmin with a 2 × 2 deg2 fully coded field-of-view, while the Compton telescope provides the angular resolution of 3◦ − 6◦ with a 60 × 60 deg2 field-of-view. The 3σ, 106 s sensitivity is expected to be about 10-5 MeV cm-2 s-1 over the entire energy range. Primary mode of observation is fixed pointing, with extended exposure of the regions of interest. However, as a standalone Compton telescope with wide FoV, Imaging Calorimeter will simultaneously provide wide-area sky exploration, significantly broadening GECCO’s observational scope. |
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Tuesday September 19 2023 | Arjen VAN DER WEL (Ghent University, Belgium)
Galaxy Evolution from z=3 to the Present Day: Recent Insights from JWST and VLT surveys Summary [click here] Thanks to an abundance of excellent observational data the evolution of the population of galaxies in terms of its stellar mass function, the distribution of star-formation across galaxies and their structural properties is known with good precision. The challenge we face, though, is to decipher the evolutionary paths of individual galaxies. Two roads to achieving this goal are 1) to reconstruct star-formation histories of individual galaxies through detailed modeling of high-quality spectra, and 2) attempt to define 'rules' for galaxy evolution that tell us how the collection of individual galaxies must evolve so that the population properties are reproduced. I will describe our recent progress along these lines, enabled by high-quality data from VLT spectroscopy of z~1 galaxies (the LEGA-C survey) and high-quality imaging data from HST (CANDELS) and JWST (CEERS). |
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Tuesday October 17 2023 | Postdoc seminar Sudip CHAKRABORTY (LEPCHE)
Unraveling the magnetic origin of dense non-equatorial ultrafast outflows in black hole X-ray binaries Summary [click here] Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era. |
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Tuesday December 5 2023 | Recent hiree seminar Barbara PERRI (LDE3)
Space weather in the era of Solar Orbiter and rising solar activity Summary [click here] Space weather is about connecting the Sun to the Earth, and being able to anticipate the most extreme solar events and their impact on our technology and health. It becomes even more challenging during periods of maximum solar activity, which happen every 11 years due to the solar dynamo, and which is currently happening right now (maximum peak estimated in 2025). Luckily, the current conjecture of space observatories dedicated to the Sun-Earth chain is very complimentary, with new missions like Solar Orbiter providing unique and useful insight.
I will present here some of my contributions to space weather research at DAp, focusing especially on the impact of solar activity and how Solar Orbiter data can help with this challenging regime. I will first explain how far-side and out of the ecliptic data gaps filled with Solar Orbiter will influence solar wind simulations. Then, I will show how the modulation of solar activity in the heliosphere will influence the propagation of transients such as CMEs (Coronal Mass Ejections) or cosmic rays. Finally, I will extend my research to the subject of exo-space weather, and discuss how the dynamo-wind coupling may influence other stars and their environment. |
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Wednesday December 6 2023 | Ute LISENFELD (University of Granada)
Molecular gas in galaxies in compact groups Summary [click here] Galaxies in compact groups live in an environment where constant interactions take place, both through tidal and through collisional processes. They are are therefore ideal objects to study the influence of the environment on galaxy properties and evolution, in particular the quenching of star formation.
The atomic hydrogen is extremely affected by this environment, with high deficiencies in many cases, whereas the molecular gas content is on average relatively normal in most galaxies. However, the properties of the molecular gas are drastically different if one selects those galaxies, based on their mid-infrared colours from the Spitzer and WISE satallites, that are in transitioning phase from active to quiescent which have have a considerably lower molecular-to-stellar mass and star formation efficiency. In my talk I will present result form observations of the molecular gas and star formation in galaxies in compact groups, both from single dish and interferometric observations, that show indications that in many cases the molecular is highly perturbed (and therefore unable to form stars) in compact groups. |
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Tuesday January 30 2024 | Florian PACAUD (University of Bonn)
Galaxy clusters and filaments in the first eROSITA all-Sky survey Summary [click here] Launched in July 2019, eROSITA was set to survey the sky in the soft X-ray band for four years and provide a much awaited update to the old ROSAT all-sky survey. In a few weeks, the collaboration will release the first all-sky map obtained from the first six month of observation. In this talk, I will present the instrument and its technical abilities. I will then explore the all-sky map and show selected results on individual local clusters and filaments. Finally, I will give a preview of the soon-to-be-released data products, including AGN and galaxy catalogs, and present some of the methods used for their cosmological analysis. |
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Tuesday February 27 2024 | Hakim ATEK (IAP)
Unveiling the faintest and the brightest galaxies at early times with JWST Summary [click here] The JWST is revolutionizing our understanding of the early Universe by unveiling a wealth of bright galaxies at z>9 and faint AGNs at z>5. I will present the latest constraints on the overabundance of UV-bright galaxies at z>9, which is 10-100 times higher than galaxy formation models. I will discuss to what extent recent theoretical efforts can reproduce such observations, and how future wide-area surveys such as Euclid will help put stronger constraints ion the bright-end of UVLF at z>8. On the other hand, faint galaxies, representing the building blocks of present-day galaxies, have eluded spectroscopic constraints, even with the deepest JWST campaigns so far. I will present the results of our UNCOVER survey, which combines ultra-deep NIRSpec spectroscopy with the strong lensing magnification of A2744 cluster. We characterize ultra-faint galaxies with intrinsic absolute magnitude between Muv=-17 and Muv=-15 at 6<z<8, and stellar masses down to 10^6 solar masses. I will discuss our plans to obtain the deepest observations on sky with the GLIMPSE program to the faintest galaxy population out to z=15 and beyond. |
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Tuesday April 2 2024 | James DUNLOP (University of Edinburgh)
Charting the emergence of the first galaxies with JWST Summary [click here] The James Webb Space Telescope (JWST) is transforming our view of galaxy formation and evolution in the young Universe. I will provide an overview of the latest results from the PRIMER survey, the largest JWST Cycle-1 “Galaxies” programme which, in combination with other public JWST imaging, is now enabling us to chart the early growth of the galaxy population back to within ~300 million years of the Big Bang. Specifically, I will present and discuss the first robust determination of the evolving galaxy luminosity function extending out to redshifts z~13, as well as new measurements of the galaxy stellar mass function reaching out to z~9. I will then attempt to interpret these results in the context of our current understanding of the evolving dark matter halo mass function and the efficiency with which galaxies are able to convert their baryons into stars. Finally, I will discuss the prospects for future progress with JWST and with the Atacama Large Millimeter Array (ALMA), including the possibility of uncovering the emergence of the first galaxies. |
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Tuesday April 30 2024 | Postdoc seminar Dinil PALAKKATHARAPPIL (LDE3)
Precise Stellar Age Determination of Stars with Gaia and TESS: Prospects and Insights for the upcoming PLATO and HAYDN Missions Summary [click here] Determining precise star ages is essential for insights into stellar, exoplanetary, and galactic evolution, but it remains a complex challenge. The two most precise techniques are (1) asteroseismology, which is the study of the internal structure of stars by means of intrinsic global oscillations, and (2) analysis of stellar clusters where stars are formed from the same molecular cloud at the same time, and the observed colour-magnitude
diagram is a strong diagnostic of the cluster age. In this talk, I will combine these two techniques to determine the age of intermediate age open cluster NGC 2477. We used astrometry and colour from Gaia to extract membership probabilities and colour information about the stars in clusters, while pulsating stars were found using TESS data. Observations are complemented with asteroseismic modelling using MESA and GYRE to obtain the precise age of the cluster. In the second part of the talk, I will detail my contribution to the development of the ground segment of the ESA M3 PLATO mission to obtain stellar mass, radius, and age as well as several rotation and magnetic activity indicators for which CEA is responsible. I will conclude by showcasing some applications of the PLATO pipeline using Kepler and TESS data. |
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Tuesday May 7 2024 | Thomas HENNING (Max Planck Institute for Astronomy, Heidelberg)
Towards New Horizons: Star and Planet Formation with JWST Summary [click here] The James Webb Space Telescope (JWST) is the most powerful space-based infrared observatory ever built. The first part of the talk will discuss the main structures of the telescope and its assembly after launch. The talk will then highlight the extremely exciting science results in the field of star and planet formation, including the nature of protostars and planet-forming disks. A special emphasis will be placed on the accretion process and the chemical building blocks of disks and what we can learn from infrared spectroscopy regarding the inventory of material in the terrestrial planet-forming zone. |
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Tuesday May 14 2024 | Recent hiree seminar Damien TURPIN (DAp)
The high-energy and multi-messenger transient sky in the eyes of the SVOM mission Summary [click here] Over the past decades, the study of highly energetic transient phenomena has led to several scientific breakthroughs from the discovery of r-process nucleosynthesis production sites to the first credible identification of ultra-high-energy cosmic-ray accelerators. In a general manner, these explosive events allow us to study extreme physical regimes (extreme gravity, extreme magnetic fields, relativistic shocks) and are unique probes to study the Universe properties up to high redshift. In the last 10 years, an unprecedented amount of new generation instruments observing the sky at all wavelengths and with all astrophysical messengers have significantly contributed to enlarging our vision of the transient sky phenomena. This observational revolution initiated by the gravitational wave interferometers has forced the astrophysical community to rapidly adapt to the new challenges of multi-messenger astronomy. In June 2024, the French community will have a leading role in this young research domain with a strong implication for the French lab. in the upcoming SVOM mission. In this seminar, I will review some of the latest results obtained by the scientific community exploring the high-energy transient sky, especially with a focus on the sources of interest for the SVOM mission. I will show how important will be the SVOM mission contribution to this exciting field of research that has probably revealed only a subset of its richness. |
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Tuesday June 11 2024 | Mustapha MEFTAH (LATMOS)
Les NanoSatellites au service de l’observation de la Terre et de l’univers Summary [click here] Le thème de la conférence portera sur l’observation de la Terre, le climat, les satellites et le NewSpace. Le 24 janvier 2021, une fusée Falcon 9 était lancée avec succès par SpaceX depuis Cap Canaveral, en Floride. Sous la coiffe du lanceur, il y avait 143 satellites. C’est à ce jour le plus grand nombre de satellites jamais déployés en une seule mission. Uvsq-Sat, premier nanosatellite français dédié à l’observation de variables climatiques essentielles, faisait partie de cette épopée. Depuis plus de trois ans, Uvsq-Sat observe la Terre et le Soleil de manière assidue. A 550 km d’altitude, ce satellite mesure la quantité d'énergie solaire réfléchie vers l'espace et la quantité d'énergie thermique émise par la Terre vers l’espace.
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Tuesday June 25 2024 | Jérôme RODRIGUEZ, Bertrand CORDIER & Damien TURPIN (DAp)
The SVOM Launch: En route for the transient hunt Summary [click here] The Space-based multi-band astronomical Variable Objects Monitor (SVOM) is a Sino-French mission planned to be launched on June 22nd 2024 from Xichang China. The launch of the satellite opens the time when scientists will step up and start exploiting the data. SVOM, however, is already a long-term project during which the involvement of engineers, developers, and scientists has already been huge. In this presentation I will overview some of the aspects of the mission, the scientific programs and aims of the project, but also recall its history and try and acknowledge the involvement of many colleagues which permitted the achievement of this fantastic machine. I will also introduce some of the novelties and specificities of the mission, in particular all the aspects dedicated to the time domain astronomy and real time analysis, implying the development of a dedicated science center, specific tools, and shifts for a certain of us as burst advocates. Finally we will (try to) have a zoom with those of us that have attended the launch in China, so that they can share with us their feeling of the launch and first impressions post launch, in this opening scientific exploitation era.
The talk will be given in French with slides in English. |
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Tuesday October 1 2024 | Recent hiree seminar Elsa DUCROT (LDE3)
Detection and characterization of rocky planets around ultra-cool stars Summary [click here] The launch of the James Webb Space Telescope (JWST) marked a significant milestone in the field of exoplanet research. For the first time, we can study the atmospheres of Earth-sized rocky planets—an opportunity that was previously unattainable. However, this is currently possible only around the coolest stars, known as red dwarfs. These stars are ideal targets for several reasons: (1) their smaller size makes it easier to detect and study small, transiting planets; (2) their lower luminosity leads to more frequent planetary transits for the same level of stellar irradiation; (3) they are the most abundant stars in the galaxy, and (4) planets orbiting them are more likely to be rocky planets with short orbital periods. In the near future, only such planetary systems are expected to produce signal-to-noise ratios (SNRs) high enough to confidently confirm or rule out the presence of atmospheres on potentially habitable planets.
In this context, my research focuses on: (1) detecting new rocky, temperate planets around red dwarfs using the SPECULOOS telescopes, and (2) observing these planets with JWST to assess the presence and composition of their atmospheres or determine the nature of their surfaces. In this talk, I will share insights into these areas of my work, including the discovery of the new SPECULOOS-3b exoplanetary system and the detailed characterization of TRAPPIST-1's innermost planets using JWST data. I will also discuss stellar contamination caused by photometric heterogeneities, which represents the primary challenge in characterizing planets around red dwarfs in transit, along with various strategies to overcome this obstacle. Finally, I will offer some perspectives on the future study of rocky planets orbiting red dwarfs. WARNING: the first minutes of the recording are missing. |
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Wednesday October 16 2024 | Daisuke NAGAI (Yale University, USA)
Cosmology in the Era of Multi-Wavelength Astronomical Surveys Summary [click here] We are entering the golden age of multi-wavelength astronomical surveys. In the 2020s, a plethora of surveys (such as Euclid, eROSITA, Rubin-LSST, Simons Observatory, and CMB-S4) are underway or planned to provide unprecedented insights into cosmology and galaxy formation. In this talk, I will provide a brief overview of significant scientific opportunities and the notable challenges in the era of big data, with highlights on recent advances in computational modeling and the integral roles played by artificial intelligence and machine learning. |
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Tuesday February 1 2022 | Pascal TREMBLIN (Maison de la Simulation)
Non-ideal self-gravity and cosmology: the importance of correlations in the dynamics of the large-scale structures of the Universe
Summary [click here] Inspired by the statistical mechanics of an ensemble of interacting particles (BBGKY hierarchy), we propose to account for small-scale inhomogeneities in self-gravitating astrophysical fluids by deriving a non-ideal Virial theorem and non-ideal Navier-Stokes equations using a decomposition of the gravitational force into a near- and far-field component. These equations involve the pair radial distribution function (similar to the two-point correlation function), similarly to the interaction energy and equation of state in liquids. Small-scale correlations lead to a non-ideal amplification of the gravitational interaction energy, whose omission leads to a missing mass problem, e.g., in galaxies and galaxy clusters. We also propose an extension of the Friedmann equations in the non-ideal regime. We estimate the non-ideal amplification factor of the gravitational interaction energy of the baryons to lie between 5 and 20, potentially explaining the observed value of the Hubble parameter. Within this framework, the acceleration of the expansion emerges naturally because of the increasing number of sub-structures induced by gravitational collapse, which increases their contribution to the total gravitational energy. A simple estimate predicts a non-ideal deceleration parameter qni~-1; this is potentially the first determination of the observed value based on an intuitively physical argument. We suggest that correlations and gravitational interactions could produce a transition to a viscous regime that can lead to flat rotation curves. This transition could also explain the dichotomy between (Keplerian) LSB elliptical galaxy and (non-Keplerian) spiral galaxy rotation profiles. Overall, our results demonstrate that non-ideal effects induced by inhomogeneities must be taken into account in order to properly determine the gravitational dynamics of galaxies and the larger scale universe. |
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Tuesday May 10 2022 | Group seminar Lev TITARCHUK (University of Ferrara, Italy)
Comptonization Problem and Its solution in Application to the Spectra of the Neutron Star and Black Hole Sources Summary [click here] In 2017 the work on the Comptonization (Sunyaev-Titarchuk) seen in the X-ray spectra of astrophysical sources was a candidate for the Nobel Prize in Physics. In this talk I provide all the details of the exciting prehistory of this topic and precise details of this discovery. The solution of this problem and its subsequent development and application to the spectra of accreting neutron star (NS) and black hole (BH) binaries reveals a lot of information on these objects. In particular, now we can unambiguously distinguish between a NS and a BH (Galactic or extragalactic) using correlations of their spectral indices vs mass accretion rate (or QPO frequency). I further demonstrate how we can determine a BH mass using this correlation. |
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Tuesday June 21 2022 | Recent hiree seminar Sandrine CODIS (DAp)
Modeling the birth and growth of the cosmic web Summary [click here] Starting from the largest scales, I will first describe how the cosmic web is woven across cosmic time into a gigantic bubble-like tapestry made of nodes, filaments, walls and voids. A particular emphasize will be put on the geometry and connectivity of this cosmic foam. Recent theoretical works aiming to precisely model the Universe on those mildly non-linear scales will be presented. In particular, I will identify a regime where large-deviation theory can be successfully implemented to predict the so-called count-in-cells statistics and describe promising cosmological applications for future galaxy surveys. The second part of the talk will focus on the birth and evolution of haloes and galaxies within these large cosmic highways. The highly anisotropic galactic environment set by the cosmic web will be shown to play a significant role in shaping them, an effect inducing large-scale galaxy alignments that are difficult to model but represent an important contamination for weak lensing experiments. |
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Tuesday January 9 2024 | Roya MOHAYAEE (IAP)
The Anomalous Dipole Summary [click here] Standard model of cosmology is based on the cosmological principle, which states that the Universe is statistically homogeneous and isotropic on large scales. Is this hypothesis supported by the observations ? After a historical survey of the field, I shall use the high redshift data from
radio galaxies and quasars to show that the early Universe does not seem to be isotropic and the rest frame of cosmic microwave background radiation does not coincide with the rest frame of distant sources. I shall also demonstrate that the cosmological principle is violated at a statistical significance of over 5-sigma. |
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Friday April 26 2024 | Joint DAp-DPhP seminar Christophe YÈCHE & Éric ARMENGAUD (DPhP)
DESI Year 1 Results - Cosmological Constraints from the Measurements of Baryon Acoustic Oscillations Summary [click here] We will present cosmological results from the measurement of baryon acoustic oscillations (BAO) in galaxy, quasar and Ly-α forest tracers from the first year of observations from the Dark Energy Spectroscopic Instrument (DESI). BAO provide robust measurements of the transverse comoving distance and Hubble rate, or their combination, relative to the sound horizon, in seven redshift bins from over 6 million extragalactic objects in the redshift range 0.1<z<4.2.
With the first six redshift bins using over 5.7 million unique galaxy and quasar redshifts measured by DESI, the combined precision of the BAO is 0.52%. The seventh BAO measurement, at redshift z~2.3 is obtained over 0.4 million Ly-α forest spectra with their auto-correlation and their cross-correlation with the spatial distribution of 0.7 million quasars. The precision for this measurement is 1.1%. Then we will present the cosmological interpretation of these BAO measurements, in the framework of the standard flat ΛCDM cosmological model. In particular we will show cosmological constraints on the Hubble parameter H0 and on the sum of neutrino mass. Finally, we will extend the interpretation to models with a constant dark energy equation of state and even to those with a time-varying dark energy equation of state. |
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Tuesday April 19 2022 | Sacha BRUN et Olivier LIMOUSIN (DAp)
Solar Orbiter: the heliospheric explorer Summary [click here] Solar Orbiter - ESA M1 Mission - has entered its scientific phase at the end of November 2021, after a cruise phase of more than 18 months (and 2.2 Billion km) and the commissioning of the 10 instruments on board. CEA/IRFU played a key role in this mission, by providing the focal plane detector array of the X-ray telescope, STIX, based on in-house Caliste technology. In the meantime, solar physicists have been busy preparing the pipelines needed to handle the data sent by the instrument, and developing high performance numerical simulations of the Sun.
In this 2-voice seminar, we will relate the first 26 months of activities of the mission, covering both STIX calibration, its first light, the first solar flare detected, as well as the development of associated numerical simulations of the Sun and optimal scientific processing of instrumental data sent as the spacecraft gets closer and closer to the Sun (last perihelion was on 26 March 2022 at 0.32 AU), while in the meantime our star is increasing in intensity and in activity, with solar magnetic cycle 25 now well on its way. |
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Tuesday January 10 2023 | Stéphane AUNE, Jean-Charles CUILLANDRE et Pierre ASTIER (CEA/SEDI, CEA/DAp et LPNHE)
20 years of MegaCam scientific operations on the Canada-France-Hawaii Telescope Summary [click here] Over its history CFHT has helped build or hosted dozens of instruments, operating from ultraviolet to mid-infrared wavelengths, attuned to Maunakea’s renowned observing conditions. One instrument, though, eclipses them all – MegaCam. It is in fact hard to imagine CFHT’s success today without the decisions made two decades ago to build what was then the largest digital focal plane in the world. MegaCam took engineering techniques to closely mount CCDs into a single camera to a whole new level. With its 1 degree field of view, MegaCam was able to tile the sky with images, creating rich and precise datasets on enormous scales that could be mined by the entire scientific community for decades. To date MegaCam has led to more peer reviewed science publications than all other instruments used at CFHT combined. It has been used to tune the model of Dark Energy and map vast regions of Dark Matter, unveil faint wisps of stars flung from interacting galaxies, reveal hundreds of free floating planets in star forming regions, and is currently directly contributing to a Stage IV dark energy measurement by providing the key ground-based photometry for the ESA Euclid space mission to derive photometric redshifts of hundreds of millions of galaxies over the northern sky. All of this is part of a revolution in wide field high-resolution optical imaging that CFHT pioneered for many years. Three speakers will offer a complete overview from technical aspects to the most impactful scientific result over these past two decades:
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Tuesday September 19 2023 | Arjen VAN DER WEL (Ghent University, Belgium)
Galaxy Evolution from z=3 to the Present Day: Recent Insights from JWST and VLT surveys Summary [click here] Thanks to an abundance of excellent observational data the evolution of the population of galaxies in terms of its stellar mass function, the distribution of star-formation across galaxies and their structural properties is known with good precision. The challenge we face, though, is to decipher the evolutionary paths of individual galaxies. Two roads to achieving this goal are 1) to reconstruct star-formation histories of individual galaxies through detailed modeling of high-quality spectra, and 2) attempt to define 'rules' for galaxy evolution that tell us how the collection of individual galaxies must evolve so that the population properties are reproduced. I will describe our recent progress along these lines, enabled by high-quality data from VLT spectroscopy of z~1 galaxies (the LEGA-C survey) and high-quality imaging data from HST (CANDELS) and JWST (CEERS). |
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Wednesday October 4 2023 | Joint DAp-DPhP seminar Roland BACON (CRAL)
WST - The Wide Field Spectroscopic Telescope Summary [click here] The WST project aim to study and built an innovative 10-m class wide-field spectroscopic survey telescope (WST) in the southern hemisphere with simultaneous operation of a large field-of-view (5 sq. degree) and high multiplex (20,000) multi-object spectrograph facility with both medium and high resolution modes (MOS), and a giant panoramic integral field spectrograph (IFS). The ambitious WST top-level requirements place it far ahead of existing and planned facilities. In just its first 5 years of operation, the MOS will target 250 million galaxies and 25 million stars at medium resolution + 2 million stars at high resolution, and 4 billion spectra with the IFS. WST will achieve transformative results in most areas of astrophysics. The combination of MOS and IFS spectroscopic surveys is one of the key aspects of the project. It is very attractive because of the high complementarity between the two approaches. I will detail this innovative point using the example of the MOS and MUSE surveys performed in the CDFS region. The project aims to be the next major post-ELT project. It is supported by a large consortium of very experienced institutes plus ESO, representing 9 European countries and Australia. |
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Tuesday November 28 2023 | Elisa CHISARI (University of Oxford)
A rising tide: galaxy alignments in the Stage IV era Summary [click here] A new generation of galaxy surveys is at our doorstep. By mapping billions of galaxies over the sky, they will deliver precision constraints on the cosmological model. The significant gains in constraining power are accompanied by the need to develop more accurate models of galaxy shapes. Several physical effects will come into play that could be neglected or simplified in previous analyses. I will discuss the constraining power of upcoming surveys, focusing mostly on Rubin Observatory, and the modelling challenges they face. Amongst such challenges, I will focus mostly on the “intrinsic alignments” of galaxies: alignments between their shapes that are likely to arise from tidal interactions between them. I will describe some new modelling strategies that could help reduce their impact on cosmological constraints. Finally, I will present some opportunities that arise for using intrinsic alignments themselves as a probe of cosmology and galaxy evolution. |
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Tuesday March 5 2024 | Recent hiree seminar Emmanuel BERTIN (LCEG)
Wide-field imaging meets deep learning: new challenges, new opportunities Summary [click here] Since the 19th century, wide-field imaging has significantly advanced numerous fields of Astrophysics, spanning from the study of solar system bodies to observational cosmology. Ongoing and future optical/near-infrared imaging surveys face many interesting data analysis challenges, especially in time domain astronomy, in a context where detector technology enable wide-field observations at increasingly high frame rates. Through concrete examples, I will show with how Deep Learning techniques offer promising solutions to address such challenges, and provide new scientific opportunities. |
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Tuesday April 30 2024 | Postdoc seminar Dinil PALAKKATHARAPPIL (LDE3)
Precise Stellar Age Determination of Stars with Gaia and TESS: Prospects and Insights for the upcoming PLATO and HAYDN Missions Summary [click here] Determining precise star ages is essential for insights into stellar, exoplanetary, and galactic evolution, but it remains a complex challenge. The two most precise techniques are (1) asteroseismology, which is the study of the internal structure of stars by means of intrinsic global oscillations, and (2) analysis of stellar clusters where stars are formed from the same molecular cloud at the same time, and the observed colour-magnitude
diagram is a strong diagnostic of the cluster age. In this talk, I will combine these two techniques to determine the age of intermediate age open cluster NGC 2477. We used astrometry and colour from Gaia to extract membership probabilities and colour information about the stars in clusters, while pulsating stars were found using TESS data. Observations are complemented with asteroseismic modelling using MESA and GYRE to obtain the precise age of the cluster. In the second part of the talk, I will detail my contribution to the development of the ground segment of the ESA M3 PLATO mission to obtain stellar mass, radius, and age as well as several rotation and magnetic activity indicators for which CEA is responsible. I will conclude by showcasing some applications of the PLATO pipeline using Kepler and TESS data. |
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Tuesday June 11 2024 | Mustapha MEFTAH (LATMOS)
Les NanoSatellites au service de l’observation de la Terre et de l’univers Summary [click here] Le thème de la conférence portera sur l’observation de la Terre, le climat, les satellites et le NewSpace. Le 24 janvier 2021, une fusée Falcon 9 était lancée avec succès par SpaceX depuis Cap Canaveral, en Floride. Sous la coiffe du lanceur, il y avait 143 satellites. C’est à ce jour le plus grand nombre de satellites jamais déployés en une seule mission. Uvsq-Sat, premier nanosatellite français dédié à l’observation de variables climatiques essentielles, faisait partie de cette épopée. Depuis plus de trois ans, Uvsq-Sat observe la Terre et le Soleil de manière assidue. A 550 km d’altitude, ce satellite mesure la quantité d'énergie solaire réfléchie vers l'espace et la quantité d'énergie thermique émise par la Terre vers l’espace.
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Tuesday September 24 2024 | Postdoc seminar Jesse PALMERIO (LISIS)
Scientific objectives of SVOM Gamma-Ray Burst follow-up with large telescopes Summary [click here] Discovered over 50 years ago, Gamma-Ray Bursts are brief flashes of high-energy photons so bright they can outshine the entire rest of the Universe during their brightest moments. These second-to-minute-long flashes, called the prompt emission, are followed by an afterglow that shines across the electromagnetic spectrum, from the radio up to TeV, and that fades over the following hours/days (or up to years in the radio). The prompt emission originates from energy dissipation within an ultra-relativistic jet produced during the formation of a stellar-mass compact object (neutron star or black hole) while the afterglow is explained as synchrotron emission from the deceleration of the jet by the surrounding medium.
GRBs have been divided into two populations (historically, based on the duration of their prompt emission, although recently this criteria is being put into question): Long GRBs (LGRBs) which are produced by the collapse of a massive star and Short GRBs (SGRBs) which are produced by the merger of two compact objects. In the last 20 years, both populations have proven themselves to be exceptional probes of the Universe, in large part thanks to dedicated follow-up campaigns designed to catch the afterglow before it fades. In this talk, I will highlight some of the recent scientific results and opportunities offered by the follow-up with large telescopes (mainly the VLT). I will discuss the implications for other astrophysical fields beyond GRB science (such as cosmology, galaxy evolution, dust, chemical enrichment, star formation…), the challenges of such studies and the prospectives for SVOM GRBs. |
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Tuesday November 12 2024 | Joel ONG (Institute for Astronomy, University of Hawaii at Manoa)
New Views into Stellar Rotation from Asteroseismology Summary [click here] Asteroseismology — the analysis and interpretation of stellar oscillations — remains the only means by which we may directly inspect the properties of stellar interiors. I will describe new theoretical developments regarding the interpretation of asteroseismic rotational signatures in a particular class of variable stars — red-giant solar-like oscillators. While waves in most variable stars propagate in a single mode cavity, I formulate a description of multicavity oscillations in these red giants by way of a physical analogy where they behave as "acoustic molecular orbitals". I describe two recent observational results which this formalism has helped us to interpret: 1. the detection of engulfment signatures in evolved red giants, and 2. direct constraints on the misalignment angle between the rotational axis of the core and of the envelope, in the central star of a spin-orbit-misaligned planetary system. |