Star formation and the cosmic evolution of the gas content of galaxies (Claudia Lagos, Durham University, UK)

We investigate the star formation and the gas content of galaxies by splitting the interstellar medium into its atomic and molecular hydrogen components, using the galaxy formation model GALFORM in the CDM framework. We calculate the ratio of molecular to atomic hydrogen, H2/HI, in each galaxy self-consistently and calculate the star formation rate (SFR) based on the H2 content. We show that the main effects of introducing a self-consistent, parameter-free SF law can be observed in the gas content of galaxies  and the location of them in the SFR vs. stellar mass plane, in a way that can be used to discriminate between the star formation laws. Without the need of reparametrization, the semi-analytic model predicts an HI mass function, CO(1-0) luminosity function, correlations between H2/HI and stellar and cold gas mass, and a far-infrared-CO luminosity relation in good agreement with local and high redshift observations. We show that the HI and H2 mass functions are characterised by radically different evolution, and that the ratio H2/HI is strongly dependent on stellar and cold gas mass, and also on redshift. The global H2/HI density ratio peaks at z~3.5, where H2 slightly dominates over HI. The predicted global density evolution of HI agrees with the observed evolution inferred from DLAs, and is always dominated by the HI content of low and intermediate mass halos.