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 HI and H-2 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 similar to 0.2 dex decrease in the HI mass of galaxies as a function of their stellar mass since z = 1.5, whereas the H-2 mass of galaxies decreases by >1 dex over the same period; (2) galaxy cold gas fractions and H-2 fractions decrease with increasing stellar mass and time. Galaxies with M-star > 10(10) M-circle dot 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 H-2 fractions; these lower fractions in combination with smaller gas reservoirs correspond to decreased present-day galaxy SFRs; (4) an H-2-based star formation relation can simultaneously fuel the evolution of the cosmic star formation and reproduce the observed weak evolution in the cosmic HI density; (5) galaxies residing in haloes with masses near 10(12) M-circle dot are most efficient at obtaining large gas reservoirs and forming H-2 at all redshifts. These two effects lie at the origin of the high star formation efficiencies in haloes with the same mass.