The Cosmic Decline in the H2/HI-Ratio in Galaxies

TL;DR

This paper models the evolution of atomic and molecular hydrogen in galaxies over cosmic time, predicting a decline in the H2/HI ratio driven by galaxy growth and pressure changes, aligning with observations.

Contribution

It introduces a pressure-based model for hydrogen phase separation in galaxies, predicting the cosmic H2/HI ratio evolution and its impact on star formation history.

Findings

Galaxies had similar HI amounts at z=1-5 as today.

H2 was substantially more abundant at high redshift.

The H2/HI ratio decreases monotonically over cosmic time.

Abstract

We use a pressure-based model for splitting cold hydrogen into its atomic (HI) and molecular (H2)components to tackle the co-evolution of HI, H2, and star formation rates (SFR) in ~3e7 simulated galaxies in the Millennium simulation. The main prediction is that galaxies contained similar amounts of HI at redshift z=1-5 than today, but substantially more H2, in quantitative agreement with the strong molecular line emission already detected in a few high redshift galaxies and approximately consistent with inferences from studies of the damped Lyman-alpha absorbers seen in the spectra of quasars. The cosmic H2/HI-ratio is predicted to evolve monotonically as Omega(H2)/Omega(HI) (1+z)^1.6. This decline of the H2/HI-ratio as a function of cosmic time is driven by the growth of galactic disks and the progressive reduction of the mean cold gas pressure. Finally, a comparison between the evolutions of HI, H2, and SFRs reveals two distinct cosmic epochs of star formation: an early epoch (z>3), driven by the evolution of Omega(HI+H2), and a late epoch (z<3), driven by the evolution of Omega(H2)/Omega(HI).