HI and H2 gas evolution over cosmic times: ColdSIM
Abstract
We present first results of the evolution of cold cosmic gas obtained through a set of state-of-the-art numerical simulations (ColdSIM). We model time-dependent atomic and molecular non-equilibrium chemistry coupled to HI and H2 self-shielding, various UV backgrounds as suggested by the recent literature, H2 dust grain catalysis, photoelectric heating, cosmic-ray heating, as well as hydrodynamics, star formation and feedback effects. By means of such non-equilibrium calculations we are finally able to reproduce the latest HI and H2 observational data. The neutral-gas mass density parameter results around neutral \!\! 10-3 and increases from lower to higher redshift (z). The molecular-gas mass density parameter shows peak values of H2 \! \! 10-4, while expected H2 fractions can be as high as 50% of the cold gas mass at z\!4-8, in line with the latest high-z measurements. Both observed HI and H2 trends are well reproduced by our non-equilibrium H2-based star formation modelling. H2 depletion times remain below the Hubble time and comparable to the dynamical time at all epochs. These findings suggest that, besides HI, non-equilibrium H2 analyses are key probes for assessing the cold gas and the role of UV background radiation. Abridged.
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