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

TL;DR

This paper introduces NeutralUniverseMachine, an empirical model that accurately describes the evolution of HI and H$_2$ gas in the universe, matching multiple observational data across redshifts.

Contribution

The model provides a novel, empirically driven framework for simulating cold gas evolution in relation to dark matter halos, improving upon previous theoretical models.

Findings

Weak evolution of HI mass function from z=0 to 3.

Strong evolution of H$_2$ mass function at the massive end.

HI bias increases with redshift, aligning with recent observations.

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 $z\sim0$, as well as the evolution of cosmic gas densities $\rho_{\rm HI}$ and $\rho_{\rm H_2}$ 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 $\tau_{\rm HI}$ from 0.1 Gyr to 10 Gyr, and the dependence of $\tau_{\rm HI}$ 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.