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 modelling of the partitioning of cold gas in galactic discs into atomic, molecular, and ionized 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 H i and H-2 in local galaxies, as well as the H i and H-2 disc sizes up to z a parts per thousand currency sign 2. We reach good agreement with the locally observed H i and H-2 mass function, although both recipes slightly overpredict the low-mass end of the H i mass function. Both of our models predict that the high-mass end of the H i 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 H-2 fraction over the entire redshift range probed than the pressure-based recipe. These strong differences in H i mass function and cosmic density between the two recipes are driven by low-mass galaxies (log (M-*/M-aS (TM)) a parts per thousand currency sign 7) residing in low-mass haloes (log (M-vir/M-aS (TM)) a parts per thousand currency sign 10). Both recipes predict that galaxy gas fractions remain high from z similar to 6to3 and drop rapidly at lower redshift. The galaxy H-2 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.