The James Webb Space Telescope has revealed the presence of spiral galaxies very early in the history of the universe (up to redshifts greater than z=5). The appearance of disks so early on is surprising, as they are fragile structures, and seems to reinforce the idea that angular momentum is contributed by the accretion of intergalactic matter. This phenomenon of accretion through cooled filaments could explain several unexpected results from the James Webb. It could also be at the origin of the universal star formation, known as secular star formation, observed in galaxies in the form of a correlation between star formation rate and stellar mass (main sequence of star formation, MS). They would provide the reservoirs for star formation and help regulate it. This represents a major paradigm shift in our understanding of the origin of galaxy shapes and their star formation history. During this thesis, we will address this question through the access to data from the James Webb, Euclid and numerical models to test this hypothesis. It should be noted that without this type of explanation for the high efficiency of galaxy formation observed by the James Webb, we would have to invoke much more drastic changes that could open up a new field. This thesis will help to determine this.

Present-day bulges of spiral galaxies and elliptical galaxies contain very old stars and are thought to be formed in the early Universe. How this actually happened in practice is not well understood, and the most relevant physical processes at play are still unclear. In the last decade, evidence has been growing of the existence of compact star bursting galaxies that might be signposts of bulges caught at the event of formation. More recently, also thanks to new findings from our group based on JWST, a number of further puzzling results has accumulated, currently difficult to explain: A) these star-bursting galaxies are always embedded in larger disk-like systems that are less active but contain most of the existing stellar mass, as if there was no ’naked’ bulge formation; B) in some cases, the outer disks have actually stopped forming stars, thus representing cases of quenching progressing from the outside-in, reversing the standard more familiar pattern (as observed in local spirals and the MilkyWay, where the center is quenched and the outskirts are forming stars); C) the disks are often strongly lopsided in their stellar mass distribution, a feature becoming more and more dominant when looking at earlier times. This phenomenology is currently unexplained. It could be related to merging activity, gas accretion or also feedback effects. If these are forming bulges, how they would evolve in present-day bulges and elliptical galaxies is unclear. Still, these new challenging observations promise breakthrough in the understanding of bulge formation if more progress can be made and further insight gathered. We propose a PhD project where the student will be using imaging and spectroscopy data from JWST to illuminate these issues. Imaging from deep and ultra deep public surveys that is accumulating will be used to increase the statistics and put on more solid grounds the early results gathered so far. The spectroscopy from JWST holds the key to detailed understanding of specific systems, providing information on kinematics of the compact star bursting cores as well as of the outer disks: if these subsystems are co-rotating without major disturbances would support non violent, gas accretion related evolution. On the contrary, counter-rotating subsystems or kinematics disturbances would betray merging events. This kind of test has not yet been carried out. We will use targeted spectroscopy in part already available from the Early-Release project CEERS of which we are members, from the large archive that is accumulating, and from dedicated proposals (pending, and to be submitted in future cycles).

This project aims to enhance the synergy between astronomical observations, numerical cosmological simulations and galaxy modelling. Upcoming instruments like Euclid, DESI and Rubin LSST, among others, will make wide-field galaxy surveys with extremely precise measurements. The enhanced precision in the observations however, will requite robust theoretical predictions from galaxy formation models to achieve a profound understanding of the fundamental physics underlying the cosmological measurements.

To achieve this, exa-supercomputers will play a key role. Unlike modern supercomputers, which typically consist of thousands of CPUs for state-of-the-art simulation productions, exa-supercomputers will employ a hybrid configuration of CPUs hosts with GPUs accelerators. This configuration will empower the computations of up to 10^18 operations per second. Exa-supercomputers will revolutionise our ability to simulate cosmological volumes spanning 4 Gigaparsecs (Gpc) with 25 trillion particles! the minimum volume and resolution requirements necessary for making predictions of Euclid data.

However, the challenge to-date lies in the fact that cosmological simulation software designed for exa-supercomputers lacks the modelling for galaxy formation. Examples include the HACC-CRKSPH code (Habib et al. 2016, Emberson et al. 2019) and PKDGRAV3 (Potter, Stadel Teyssier 2017), that have produced the largest simulations to-date, FarPoint (Frontiere et al. 2022), encompassing 1.86 trillion particles within a 1 Gpc volume, and Euclid Flagship (Potter, Stadel Teyssier 2017), featuring 2 trillion particles in a 3 Gpc volume, respectively. While HACC-CRKSPH and PKDGRAV3 were developed to run on modern GPUs-accelerated supercomputers, they lack the complex physics of galaxy formation and can therefore only produce gravity-only cosmological boxes.

The SWIFT code (Schaller et al. 2023) is a parallel effort that has produced Flamingo (Schaye et al. 2023), the largest simulation that integrates gravity, hydrodynamics and galaxy formation physics, encompassing 0.3 trillion particles. However, the caveat of SWIFT is that it was primarily designed for CPU usage. The adaptation of SWIFT to run on modern GPUs will require the entire redevelopment of the code. Another example are the current big simulations of galaxy formation done at Irfu, such as Extreme Horizon (Chabanier et al. 2020), that have also reached their limit as they rely on CPU-based codes that hamper their scalability.

Understanding the intricacies of galaxy formation is paramount for interpreting astronomical observations. In this pursuit, CEA DRF/Irfu stands uniquely positioned to lead the advances in astrophysics in the emerging exascale era. Researchers at DAp and DPhP have already embarked on the analysis of high-quality data from the Euclid mission and DESI. Simultaneously, a team at DEDIP is developing DYABLO (Durocher Delorme, in preparation), a robust gravity + hydrodynamics code tailored explicitly for exa-supercomputing.

In recent years, significant investments have been channeled into the advancement of DYABLO. Numerous researchers at DAp and DEDIP have contributed on various aspects (from the hydrodynamics of solar physics to refining Input/Output processes) thanks to collaborative grants such as PTC-CEA grant and FETHPC European project IO-SEA. Additionally, DYABLO has benefited from interactions with CEA research unit, Maison de la simulation (CEA CNRS).

This ambitious project aims to extend DYABLOs capabilities by integrating galaxy formation modules in collaboration with Maxime Delorme. These modules will encompass radiative gas cooling and heating, star formation, chemical enrichment, stellar mass loss, energy feedback, black holes, and active galactic nuclei feedback. The ultimate objective is to enhance the analysis of Euclid and DESI data by generating simulation predictions of galaxy formation and evolution using DYABLO. The initial dataset will involve a comprehensive examination of clustering of matter and galaxy clustering, in partnership with researchers at DAp/LCEG and DAp/CosmoStat.

This thesis will create the first version of a galaxy formation code optimised for exa-scale supercomputing. Ongoing developments will not only expand its capabilities but also unlock new opportunities for in-depth research, enhancing synergy between astronomical observations, numerical cosmological simulations, and galaxy modelling.

References:
Habib, S., et al., 2016, New Astronomy, Volume 42, p. 49-65.
Emberson, J.D., et al., 2019, The Astrophysical Journal, Volume 877, Issue 2, article id. 85, 17 pp.
Potter, D., Stadel, J., Teyssier, R., 2017, Computational Astrophysics and Cosmology, Vol. 4, Issue 1, 13 pp.
Frontiere, N., et al., 2023, The Astrophysical Journal Supplement Series, Volume 264, Issue 2, 24 pp.
Schaller, M., et al., 2023, eprint arXiv:2305.13380
Schaye, J., et al., 2023, eprint arXiv:2306.04024
Chabanier, S., et al., 2020, Astronomy Astrophysics, Volume 643, id. L8, 12 pp.

Clusters of galaxies are the most massive entities in the universe.
Applying artificial intelligence to the cosmological analysis of X-ray cluster surveys allows us to tackle this problem from a totally new perspective. Only directly observable parametres are used (redshift, X-ray colour and flux) in a deep learning approach based on hydrodynamical simulations; this allows us to establish an implicit link between the X-ray parameters and the underlying dark matter distribution. From this, we can infer the cosmological parameters, without explicitly computing cluster masses and bypassing the empirical formalism of scaling relations between the X-ray properties and cluster masses.
The goal of the thesis is to apply this method (developed at DAP) to the XMM-XXL survey, which is, 24 years after the XMM launch, the only programme having assembled a cosmological cluster sample with controlled selection effects (~ 400 objects). The expected results will constitute a first in the history of observational cosmology.

The James Webb Space Telescope (JWST) is revolutionizing our view of the first billion years after the big bang, by enabling us to detect the primordial galaxies formed by the collapse of the Universes first overdensities. Initial studies of the properties of these galaxies, partly carried out by our team, have revealed that their formation is still largely misunderstood and potentially in tension with the Lambda Cold Dark Matter (LCDM) model. Indeed, these studies have uncovered a potential excess of massive primordial galaxies, implying accelerated growth of these galaxies at star formation efficiencies well beyond the predictions of theoretical models. Before invoking radically different cosmological and galaxy evolution models, however, it is necessary to confirm these tensions, which are currently based only on highly uncertain measurements of the stellar mass of a few galaxies.
The aim of this thesis is to confirm or refute these tensions by accurately constraining, for the first time, the stellar mass of a large statistical sample of primordial galaxies. To do this, we will combine data from four JWST extragalactic surveys with an original statistical approach of image stacking, enabling us to obtain the average stellar mass of primordial galaxies that are otherwise too faint to be detected individually by the JWST in the critical mid-infrared window. This information, together with that obtained on their star-forming activity, will be decisive in understanding the growth of the Universes first galaxies.