Research Highlight

Neutral gas heating by X-rays in primitive galaxies: Infrared observations of IZw18 with Herschel
[ADS link]
The neutral interstellar medium of galaxies acts as a reservoir to fuel star formation. The dominant heating and cooling mechanisms in this phase are uncertain in extremely metal-poor star-forming galaxies. The low dust-to-gas mass ratio and low polycyclic aromatic hydrocarbon abundance in such objects suggest that the traditional photoelectric effect heating may not be effective.
Our objective is to identify the dominant thermal mechanisms in one such galaxy, IZw18 (1/30 solar metallicity), assess the diagnostic value of fine-structure cooling lines, and estimate the molecular gas content. Even though molecular gas is an important catalyst and tracer of star formation, constraints on the molecular gas mass remain elusive in the most metal-poor galaxies.
Building on a previous photoionization model describing the giant HII region of IZw18-NW within a multi-sector topology, we provide additional constraints using, in particular, the [CII] 157µm and [OI] 63µm lines and the dust mass recently measured with the Herschel Space Telescope.
The heating of the HI region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while the photoelectric effect is negligible. Significant cosmic ray heating is not excluded. Inasmuch as X-ray heating dominates in the HI gas, the infrared fine-structure lines provide an average X-ray luminosity of order 4×10^40 erg s−1 over the last few 10^4 years in the galaxy. The upper limits to the [NeV] lines provide strong constraints on the soft X-ray flux arising from the binary. A negligible mass of H2 is predicted. Nonetheless, up to ~10^7 solar masses of H2 may be hidden in a few sufficiently dense clouds of order ~10 pc in size. Regardless of the presence of significant amounts of H2 gas, [CII] and [OI] do not trace the so-called “CO-dark gas”, but they trace the almost purely atomic medium. Although the [CII]+[OI] to total infrared ratio in IZw18 is similar to values in more metal-rich sources (~1%), it cannot be safely used as a photoelectric heating efficiency proxy. This ratio seems to be kept stable owing to a correlation between the X-ray luminosity and the star formation rate.
X-ray heating could be an important process in extremely metal-poor sources. The lack of photoelectric heating due to the low dust-to-gas ratio tends to be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation.

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Research Interests

  • Chemical evolution of galaxies
  • Star-formation of history in galaxies
  • Thermal processes in the interstellar medium
  • Properties of the interstellar molecules and dust

List of refereed publications


  • 2014-: CNRS (CR1) researcher at Laboratoire AIM, CEA-Saclay (France)
  • 2013: Research associate at Cornell University (USA)
    Development of the high-resolution spectral atlas CASSIS for Spitzer/IRS
  • 2013: Research associate at Service d'Astrophysique, CEA-Saclay (France)
  • 2013: Research associate at LUTH, Paris-Meudon Observatory (France)
  • 2010-2013: CEA Eurotalents fellow at Service d'Astrophysique, CEA-Saclay (France)
    Herschel Space Telescope; SAG2
  • 2005-2010: Research associate at Cornell University (USA)
    Spitzer Space Telescope; IRS instrument
    Development of the low-resolution spectral atlas CASSIS for Spitzer/IRS
  • 2002-2005: Ph.D. at the Institut d'Astrophysique de Paris (France)
    FUSE telescope; chemical evolution of galaxies
    Supervisor: D. Kunth


Below you can find some of the projects I'm involved in. Make sure to use the "browse" menu on top to access the full content.

Atacama Large Millimeter Array (ALMA)

  • PI Cycle 1&2 proposal to observe LMC-N11; several co-I proposals

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Herschel (far-infrared space telescope)

  • SAG2 member: DGS (PI Madden), SHINING (PI Sturm), VNGS (PI Wilson), HRS (PI Eales)
  • Specific interests: Low-metallicity dwarf galaxies, ultraluminous Infrared Galaxies (ULIRGs), photodissociation regions (PDRs)
  • PI of 2 proposals; several co-I proposals, including HERITAGE (PI Meixner) & HERUS (PI Farrah)
  • Specific tools: PI PACSman
  • Other tools used: HIPE

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Spitzer (mid-infrared space telescope)

  • Member of Spitzer/IRS PI team at Cornell University
  • Specific interests: Low-metallicity dwarf galaxies, giant HII regions, photodissociation regions (PDRs)
  • PI of 4 proposals, several co-I proposals
  • Specific tools; PI CASSIS spectral atlas & SMART suite of tools
  • Other tools used: spectral maps with CUBISM

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Hubble (optical/UV space telescope)

  • PI program 13697 (9 orbits for low-metallicity dwarf galaxies with COS)

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Stratospheric Observatory for Infrared Astronomy (SOFIA)

  • Specific interests: Magellanic Clouds, photodissociation regions (PDRs)
  • PI proposal with GREAT instrument; several co-I proposals

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Far Ultraviolet Spectroscopic Explorer (FUSE)

  • Specific interests: Low-metallicity dwarf galaxies, chemical evolution
  • Several co-I proposals

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Other observatories


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Below you can find some of the tools I developed. Make sure to use the "browse" menu on top to access the full content.


CASSIS is the spectral atlas for the Spitzer/IRS instrument. It contains all the high- and low-resolution spectra observed by the IRS in staring mode.

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SMART is an IDL package to reduce, extract, and analyze Spitzer/IRS data. It includes the extraction algorithm AdOpt for complex sources (multiple, complex background...).

Check it out!


PACSman is a tool to extract line profiles from the Herschel/PACS instrument, project spectral maps, perform optimal extraction on point sources etc...

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I am interested and more or less involved in the following possible future missions.


SPICA is an ESA/JAXA infrared mission proposed for M5.

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LUVOIR is a proposed NASA flagship mission. I am interested in the potential implementation of a high-resolution UV spectropolarimeter.

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Misc. Stuff

Other things...

FAQ for PhD & Interns

Link to a document, in French/English, that summarizes the important step during a PhD, the differences with an internship, the potential problems, how to prepare the after PhD etc...


Research Environment

Some documents and guides to build a nice research environment (email, programming, bibliography etc...).


Les frères Ozone

Website in French to explain ozone and the ozone layer to kids (master project in a week-long communication course).


"Strange" paper titles

A small list of rather strange paper titles I've come across on arXiv...