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

Abstract

We present a semi-empirical model to infer the atomic and molecular hydrogen\ncontent of galaxies as a function of halo mass and time. Our model combines the\nSFR-halo mass-redshift relation (constrained by galaxy abundances) with\ninverted SFR-surface density relations to infer galaxy H I and H2 masses. We\npresent gas scaling relations, gas fractions, and mass functions from z = 0 to\nz = 3 and the gas properties of galaxies as a function of their host halo\nmasses. Predictions of our work include: 1) there is a ~ 0.2 dex decrease in\nthe H I mass of galaxies as a function of their stellar mass since z = 1.5,\nwhereas the H2 mass of galaxies decreases by > 1 dex over the same period. 2)\ngalaxy cold gas fractions and H2 fractions decrease with increasing stellar\nmass and time. Galaxies with M* > 10^10 Msun are dominated by their stellar\ncontent at z < 1, whereas less-massive galaxies only reach these gas fractions\nat z = 0. We find the strongest evolution in relative gas content at z < 1.5.\n3) the SFR to gas mass ratio decreases by an order of magnitude from z = 3 to z\n= 0. This is consistent with lower H2 fractions; these lower fractions in\ncombination with smaller gas reservoirs correspond to decreased present-day\ngalaxy SFRs. 4) an H2-based star- formation relation can simultaneously fuel\nthe evolution of the cosmic star-formation and reproduce the observed weak\nevolution in the cosmic HI density. 5) galaxies residing in haloes with masses\nnear 10^12 Msun are most efficient at obtaining large gas reservoirs and\nforming H2 at all redshifts. These two effects lie at the origin of the high\nstar-formation efficiencies in haloes with the same mass.\n