A halo model for cosmological neutral hydrogen: abundances and
  clustering
  We extend the results of previous analyses towards constraining the abundance
and clustering of post-reionization ($z \sim 0-5$) neutral hydrogen (HI)
systems using a halo model framework. We work with a comprehensive HI dataset
including the small-scale clustering, column density and mass function of HI
galaxies at low redshifts, intensity mapping measurements at intermediate
redshifts and the UV/optical observations of Damped Lyman Alpha (DLA) systems
at higher redshifts. We use a Markov Chain Monte Carlo (MCMC) approach to
constrain the parameters of the best-fitting models, both for the HI-halo mass
relation and the HI radial density profile. We find that a radial exponential
profile results in a good fit to the low-redshift HI observations, including
the clustering and the column density distribution. The form of the profile is
also found to match the high-redshift DLA observations, when used in
combination with a three-parameter HI-halo mass relation and a redshift
evolution in the HI concentration. The halo model predictions are in good
agreement with the observed HI surface density profiles of low-redshift
galaxies, and the general trends in the the impact parameter and covering
fraction observations of high-redshift DLAs. We provide convenient tables
summarizing the best-fit halo model predictions.
