The halo occupation distribution of HI from 21-cm 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 versus 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 21-cm 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 Square Kilometre Array Pathfinder to HOD parameters. We find that the power spectrum of redshifted 21-cm intensity could be used to study the distribution of HI within dark matter haloes at z greater than or similar to 0.5 where individual galaxies cannot be detected. In addition to the HOD of HI, the amplitude of the 21-cm power spectrum would also yield estimates of the cosmic HI content (Omega(HI)) at epochs between the local Universe and redshifts probed by damped Ly alpha absorbers.