Cosmic evolution of the atomic and molecular gas content of galaxies
  We study the evolution of the cold gas content of galaxies by splitting the
interstellar medium into its atomic and molecular hydrogen components, using
the galaxy formation model GALFORM in the LCDM framework. We calculate the
molecular-to-atomic hydrogen mass ratio, H2/HI, in each galaxy using two
different approaches; the pressure-based empirical relation of Blitz &
Rosolowsky and the theoretical model of Krumholz, McKeee & Tumlinson, and apply
them to consistently calculate the star formation rates of galaxies. We find
that the model based on the Blitz & Rosolowsky law predicts an HI mass
function, CO(1-0) luminosity function, correlations between the H2/HI ratio and
stellar and cold gas mass, and infrared-CO luminosity relation in good
agreement with local and high redshift observations. The HI mass function
evolves weakly with redshift, with the number density of high mass galaxies
decreasing with increasing redshift. In the case of the H2 mass function, the
number density of massive galaxies increases strongly from z=0 to z=2, followed
by weak evolution up to z=4. We also find that the H2/HI ratio of galaxies is
strongly dependent on stellar and cold gas mass, and also on redshift. The
slopes of the correlations between H2/HI and stellar and cold gas mass hardly
evolve, but the normalisation increases by up to two orders of magnitude from
z=0-8. The strong evolution in the H2 mass function and the H2/HI ratio is
primarily due to the evolution in the sizes of galaxies and secondarily, in the
gas fractions. The predicted cosmic density evolution of HI agrees with the
observed evolution inferred from DLAs, and is dominated by low/intermediate
mass halos. We find that previous theoretical studies have largely
overestimated the redshift evolution of the global H2/HI ratio due to limited
resolution. We predict a maximum of rho_H2/rho_HI~1.2 at z~3.5.
