NeutralUniverseMachine: An Empirical Model for the Evolution of HI and H${}_2$ Gas in the Universe

Abstract

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 HI and H${}_2$ gas along with dark matter halos based on the UniverseMachine catalog. It is able to accurately describe the observed HI and H${}_2$ mass functions, molecular-to-atomic ratio, HI-halo mass relation, HI/H${}_2$-stellar mass relations at $z\sim 0$, as well as the evolution of cosmic gas densities $\ddot{I}{}_{\mathrm{HI}}$ and $\ddot{I}{}_{{\mathrm{H}}_2}$ in $0<z<6$. The predictions from our model include: (i) There is weak evolution of HI mass function in $0<z<3$, but the evolution of H${}_2$ mass function is much stronger at the massive end. (ii) The average HI 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 varying HI depletion time $\ddot{I}{„}_{\mathrm{HI}}$ from 0.1 Gyr to 10 Gyr, and the dependence of $\ddot{I}{„}_{\mathrm{HI}}$ on stellar mass and redshift is much stronger than those of H${}_2$ depletion time. The quenched galaxies have much longer gas depletion time and weaker redshift evolution. (iv) The cosmic baryon density associated with galaxies is dominated by stars for $z<1.2$ and mainly contributed by HI gas at higher redshifts. (v) The HI bias gradually increases with the redshift from 0.69 to 2.33 in $0<z<3$ and is consistent with recent HI intensity mapping experiments.

28 pages, 21 figures. Accepted for publication in ApJ. Catalogs for gas masses available at https://halos.as.arizona.edu/UniverseMachine/DR1/Gas_Masses_NeutralUniverseMachine/