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 (H-2) components to tackle the co-evolution of H I, H-2, and star formation rates (SFR) in similar to 3 x 10(7) simulated galaxies in the Millennium simulation. The main prediction is that galaxies contained similar amounts of Hi at redshift z approximate to 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 H-2/H I ratio is predicted to evolve monotonically as Omega(H2)/Omega(HI) alpha (1 + z)(1.6). This decline of the H-2/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, H-2, and SFRs reveals two distinct cosmic epochs of star formation: an early epoch (z greater than or similar to 3), driven by the evolution of Omega(HI+H2) (z), and a late epoch (z less than or similar to 3), driven by the evolution of Omega(H2)(z)/Omega(HI)(z).