Which galaxies dominate the neutral gas content of the Universe?
  We study the contribution of galaxies with different properties to the global
densities of star formation rate (SFR), atomic (HI) and molecular hydrogen (H2)
as a function of redshift. We use the GALFORM model of galaxy formation, which
is set in the LCDM framework. This model includes a self-consistent calculation
of the SFR, which depends on the H2 content of galaxies. The predicted SFR
density and how much of this is contributed by galaxies with different stellar
masses and infrared luminosities are in agreement with observations. The model
predicts a modest evolution of the HI density at z<3, which is also in
agreement with the observations. The HI density is predicted to be always
dominated by galaxies with SFR<1Msun/yr. This contrasts with the H2 density,
which is predicted to be dominated by galaxies with SFR>10Msun/yr. Current
high-redshift galaxy surveys are limited to detect carbon monoxide in galaxies
with SFR>30Msun/yr, which in our model make up, at most, 20% of the H2 in the
universe. In terms of stellar mass, the predicted H2 density is dominated by
massive galaxies, Mstellar>10^10Msun, while the HI density is dominated by low
mass galaxies, Mstellar<10^9Msun. In the context of upcoming neutral gas
surveys, we suggest that the faint nature of the galaxies dominating the HI
content of the Universe will hamper the identification of optical counterparts,
while for H2, we expect follow up observations of molecular emission lines of
already existing galaxy catalogues to be able to uncover the H2 density of the
Universe.
