| Summary: | Accurately modeling the cold gas content in the universe is challenging for current theoretical models. We propose a new empirical model NeutralUniverseMachine for the evolution of H i and H _2 gas along with dark matter halos based on the UniverseMachine catalog. It is able to accurately describe the observed H i and H _2 mass functions, molecular-to-atomic ratio, H i –halo mass relation, H i /H _2 –stellar mass relations at z ∼ 0, as well as the evolution of cosmic gas densities ρ _H I and ${\rho }_{{{\rm{H}}}_{2}}$ at 0 < z < 6. The predictions from our model include the following: (i) There is weak evolution of H i mass function at 0 < z < 3, but the evolution of H _2 mass function is much stronger at the massive end. (ii) The average H i and H _2 masses at a given stellar mass decrease by around 1 dex since z = 3 for the star-forming galaxies, but the evolution for the quenched galaxies is much weaker. (iii) Star-forming galaxies have a varying H i depletion time τ _H I from 0.1–10 Gyr, and the dependence of τ _H I on stellar mass and redshift is much stronger than those of the H _2 depletion time. The quenched galaxies have a much longer gas depletion time and weaker redshift evolution. (iv) The cosmic baryon density associated with galaxies is dominated by stars at z < 1.2 and mainly contributed by H i gas at higher redshifts. (v) The H i bias gradually increases with the redshift from 0.69 to 2.33 at 0 < z < 3 and is consistent with recent H i intensity mapping experiments.
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