2009ApJ...696L.129O

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 ∼3x10⁷ 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. 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, and a late epoch (z ≲ 3), driven by the evolution of.