Abstract
We present first results of cold cosmic gas evolution obtained through a set of state-of-the-art numerical simulations (ColdSIM). We model time-dependent atomic and molecular non-equilibrium chemistry coupled to hydrodynamics, star formation, feedback effects, various UV backgrounds as suggested by the recent literature, HI and H2 self-shielding, H2 dust grain catalysis, photoelectric heating and cosmic-ray heating. By means of such nonequilibriumcalculations we are finally able to reproduce the latest HI and H2 observational data. Consistently with available determinations, neutral-gas mass density parameter results around Ωneutral ∼ 10−3 and increases from lower to higher redshift (z). The molecular-gas mass density parameter shows peak values of ΩH2 ∼ 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 measurements from high-z galaxies. These values agree with observations up to z ∼ 7 and both HI and H2 trends are well reproduced by our non-equilibrium H2-based star formation modelling. Corresponding H2 depletion times remain below the Hubble time and comparable to the dynamical time at all epochs. This implies that non-equilibrium molecular cooling is efficient at driving cold-gas collapse in a variety of environments and since the first half Gyr. Our findings suggest that, besides HI, non-equilibrium H2 analyses are key probes for assessing cold gas and the role of UV background radiation.
Highlights
Atomic and molecular gas are fundamental phases of the cosmic medium since they constitute the bulk of neutral cosmic gas and lead to star formation and cosmic structure evolution
We model time-dependent atomic and molecular non-equilibrium chemistry coupled to hydrodynamics, star formation, feedback effects, various UV backgrounds as suggested by the recent literature, HI and H2 self-shielding, H2 dust grain catalysis, photoelectric heating and cosmic-ray heating
A pictorial view of the simulations performed can be seen in figure 1, where maps of gas entropy, HI and H2 fractions are shown at z = 4.9, after the HM UV background sets in
Summary
Atomic and molecular gas are fundamental phases of the cosmic medium since they constitute the bulk of neutral cosmic gas and lead to star formation and cosmic structure evolution. Neutral gas is constituted by gas with temperatures below 104 K (i.e. cold gas), while its chemical composition is characterised by neutral H atoms (HI), as well as by large amounts of H2 molecules, the most abundant ones in the Universe These latter drive gas collapse and its conversion into stars. Feedback effects from newly born structures, act on existing gas via a number of additional processes, such as supernova (SN) explosions, winds, metal enrichment, UV photoionization or photodissociation. These can affect HI and H2 evolution dramatically.
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