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Evolution of the atomic and molecular gas content of galaxies

We study the evolution of atomic and molecular gas in galaxies in semi-analytic models of galaxy formation that include new modeling of the partitioning of cold gas in galactic discs into atomic, molecular, and ionised phases. We adopt two scenarios for the formation of molecules: one pressure-based and one metallicity-based. We find that both recipes successfully reproduce the gas fractions and gas-to-stellar mass ratios of HI and H2 in local galaxies,as well as the HI and H2 disc sizes up to z<2. We reach good agreement with the locally observed HI and H2 mass function, although both recipes slightly overpredict the low-mass end of the HI mass function. Both of our models predict that the high-mass end of the HI mass function remains nearly constant at redshifts z < 2.0. The metallicity-based recipe yields a higher cosmic density of cold gas and much lower cosmic H2 fraction over the entire redshift range probed than the pressure based recipe. These strong differences in HI mass function and cosmic density between the two recipes are driven by low mass galaxies (log (M*/Msun) < 7) residing in low mass halos (log (Mvir/Msun) < 10). Both recipes predict that galaxy gas fractions remain high from z ~ 6-3 and drop rapidly at lower redshift. The galaxy H2 fractions show a similar trend, but drop even more rapidly. We provide predictions for the CO J = 1-0 luminosity of galaxies, which will be directly comparable with observations with sub-mm and radio instruments.