THE COSMIC DECLINE IN THE H2/H I RATIO IN GALAXIES

We use a pressure-based model for splitting cold hydrogen into its atomic (H I) and molecular (H2) components to tackle the co-evolution of H I, H2, and star formation rates (SFR) in ∼3 x 107 simulated galaxies in the Millennium simulation. The main prediction is that galaxies contained similar amounts of H I 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-α absorbers seen in the spectra of quasars. The cosmic H2/H I ratio is predicted to evolve monotonically as ΩH2/ΩHI∝(1+z)1.6. This decline of the H2/H I 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 H I, H2, and SFRs reveals two distinct cosmic epochs of star formation: an early epoch (z ∼> 3), driven by the evolution of ΩHI+H2(z), and a late epoch (z ∼< 3), driven by the evolution of ΩH2(z)/ΩHI(z).