Evolution of the atomic and molecular gas content of galaxies in dark matter haloes

We present a semi-empirical model to infer the atomic and molecular hydrogen content of galaxies as a function of halo mass and time. Our model combines the star formation rate (SFR)-halo mass-redshift relation (constrained by galaxy abundances) with inverted SFR-surface density relations to infer galaxy H I and H2 masses. We present gas scaling relations, gas fractions, and mass functions from z = 0 to 3 and the gas properties of galaxies as a function of their host halo masses. Predictions of our work include: (1) there is an ˜0.2 dex decrease in the H I mass of galaxies as a function of their stellar mass since z = 1.5, whereas the H2 mass of galaxies decreases by >1 dex over the same period; (2) galaxy cold gas fractions and H2 fractions decrease with increasing stellar mass and time. Galaxies with M⋆ > 1010 M⊙ are dominated by their stellar content at z ≤ 1, whereas less-massive galaxies only reach these gas fractions at z = 0. We find the strongest evolution in relative gas content at z <1.5; (3) the SFR to gas mass ratio decreases by an order of magnitude from z = 3 to 0. This is accompanied by lower H2 fractions; these lower fractions in combination with smaller gas reservoirs correspond to decreased present-day galaxy SFRs; (4) an H2-based star formation relation can simultaneously fuel the evolution of the cosmic star formation and reproduce the observed weak evolution in the cosmic H I density; (5) galaxies residing in haloes with masses near 1012 M⊙ are most efficient at obtaining large gas reservoirs and forming H2 at all redshifts. These two effects lie at the origin of the high star formation efficiencies in haloes with the same mass