Modelling neutral hydrogen in galaxies using cosmological hydrodynamical
  simulations
  The characterisation of the atomic and molecular hydrogen content of
high-redshift galaxies is a major observational challenge that will be
addressed over the coming years with a new generation of radio telescopes. We
investigate this important issue by considering the states of hydrogen across a
range of structures within high-resolution cosmological hydrodynamical
simulations. Additionally, our simulations allow us to investigate the
sensitivity of our results to numerical resolution and to sub-grid baryonic
physics (especially feedback from supernovae and active galactic nuclei). We
find that the most significant uncertainty in modelling the neutral hydrogen
distribution arises from our need to model a self-shielding correction in
moderate density regions. Future simulations incorporating radiative transfer
schemes will be vital to improve on our empirical self-shielding threshold.
Irrespective of the exact nature of the threshold we find that while the atomic
hydrogen mass function evolves only mildly from redshift two to zero, the
molecular hydrogen mass function increases with increasing redshift, especially
at the high-mass end. Interestingly, the weak evolution of the neutral hydrogen
mass function is insensitive to the feedback scheme utilised, but the opposite
is true for the molecular gas, which is more closely associated with the star
formation in the simulations.
