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14 juin 2019 Tout le mois


Vianney Lebouteiller (Laboratoire AIM - CEA Saclay) - The interstellar medium of nearby primitive galaxies

Vendredi 14 juin 11:00-12:00

The lack of detection of cold molecular gas in blue compact dwarf (BCD) galaxies is at variance with their intense star-formation episode. In particular, CO, often used a tracer of H2 through a conversion function, is selectively photodissociated in dust-poor environments. A potentially large fraction of H2 is thus expected to reside in the so-called CO-free gas, where it could be traced by neutral gas observed with infrared cooling lines [CI], [CII], and [OI]. Although the fraction of CO-free gas to total molecular gas is expected to be relatively large in metal-poor galaxies, a definite evidence is still lacking because of the difficulty in associating cooling lines with any given heating mechanism. The main issue at stake is to understand the role of molecular gas in the star formation process.

I will first show that the heating mechanism in the neutral gas cannot be dominated by the photoelectric effect on dust grains below a threshold metallicity due to a low abundance of dust and polycyclic aromatic hydrocarbons. I will then present results from a study on the dwarf galaxy IZw18 ( 2% solar metallicity) recently published in Lebouteiller et al. (2017). Optical and infrared lines are used to constrain the physical conditions in the HII region + HI region within a consistent photoionization and photodissociation model. We show that the HI region is entirely heated by a single ultraluminous X-ray source with important consequences on the applicability of [CII] to trace the star-formation rate and to trace the CO-free gas. We derive stringent upper limits on the size of H2 clumps that may be detected in the future with JWST and IRAM/NOEMA. We also show that the nature of the X-ray source can be constrained through the use of our models. I will conclude by proposing that star formation may be quenched in extremely metal-poor dwarf galaxies due to X-ray photoionization.

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