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Star formation in low gas density and low metallicity environments

Abstract : In nearby galaxies, an empirical relation has been established between star formation and gas surface densities, the Kennicutt-Schmidt (KS). The relation is nearly linear when molecular gas (H2) is considered, while is less tight with atomic hydrogen (HI). These low gas density regions are of a key importance in the field of star formation, since the are also low metallicity environments, resembling the conditions of a younger universe. This thesis summarizes the observational work done with the IRAM 30MT telescope in two kinds of such regions : disk galaxies with extended ultra-violet emission (XUV), and the interstellar medium going into the hot intra-cluster medium (ICM) under ram-pressure stripping in the Virgo cluster. The galex telescope has unveiled in far ultra-violet (FUV) star formation in the outer parts of some disk galaxies that was not traced by HA. To determine the presence of H2 and analyse the K-S relation in these regions, CO observations where done in the outskirts of several XUV disk galaxies, finding both detections and upper limits. These K-S relations showed a broken power law at low gas densities, below the HI-H2 threshold. In the Virgo cluster, similar CO observations were done along the HI tidal arm connecting NGC4388 and M86, where no H2 is expected. Two detections were found, showing very low star formation efficiencies (depleting less than 0,1 % of the gas reservoir per 10 [exposant] 8 yr), and showing again a disconuity of the K-S relation at low gas densities, probing that the process of a gas consumption into stars well known at high densities cannot be extrapolated to lower densities, and that H2 can survive a certain time in the hostile ICM.
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Celia Anahi Verdugo Salgado. Star formation in low gas density and low metallicity environments. Astrophysics [astro-ph]. Observatoire de Paris, 2015. English. ⟨tel-02095302⟩

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