2020MNRAS.497.5008S

Modelling the molecular gas that is routinely detected through CO observations of high-redshift galaxies constitutes a major challenge for ab initio simulations of galaxy formation. We carry out a suite of cosmological hydrodynamic simulations to compare three approximate methods that have been used in the literature to track the formation and evolution of the simplest and most abundant molecule, H₂. Namely, we consider (i) a semi-empirical procedure that associates H₂ to dark-matter haloes based on a series of scaling relations inferred from observations, (ii) a model that assumes chemical equilibrium between the H₂ formation and destruction rates, and (iii) a model that fully solves the out-of-equilibrium rate equations and accounts for the unresolved structure of molecular clouds. We study the impact of finite spatial resolution and show that robust H₂ masses at redshift z ≃ 4 can only be obtained for galaxies that are sufficiently metal enriched in which H₂ formation is fast. This corresponds to H₂ reservoirs with masses M_H2_≳6×10^9 M_☉. In this range, equilibrium and non-equilibrium models predict similar molecular masses (but different galaxy morphologies) while the semi-empirical method produces less H₂. The star formation rates as well as the stellar and H₂ masses of the simulated galaxies are in line with those observed in actual galaxies at similar redshifts that are not massive starbursts. The H₂ mass functions extracted from the simulations at z ≃ 4 agree well with recent observations that only sample the high-mass end. However, our results indicate that most molecular material at high z lies yet undetected in reservoirs with 10^9<M_H2_< 10¹⁰ M_☉.