The Halo Occupation Distribution of HI from 21cm Intensity Mapping at
  Moderate Redshifts
  The spatial clustering properties of HI galaxies can be studied using the
formalism of the halo occupation distribution (HOD). The resulting parameter
constraints describe properties like gas richness verses environment.
Unfortunately, clustering studies based on individual HI galaxies will be
restricted to the local Universe for the foreseeable future, even with the
deepest HI surveys. Here we discuss how clustering studies of the HI HOD could
be extended to moderate redshift, through observations of fluctuations in the
combined 21cm intensity of unresolved galaxies. In particular we make an
analytic estimate for the clustering of HI in the HOD. Our joint goals are to
estimate i) the amplitude of the signal, and ii) the sensitivity of telescopes
like the Australian SKA Pathfinder to HOD parameters. We find that the power
spectrum of redshifted 21cm intensity could be used to study the distribution
of HI within dark matter halos at z>0.5 where individual galaxies cannot be
detected. In addition to the HOD of HI, the amplitude of the 21cm power
spectrum would also yield estimates of the cosmic HI content at epochs between
the local Universe, and redshifts probed by damped Ly-alpha absorbers.
