The halo occupation distribution of H i from 21-cm intensity mapping at moderate redshifts

Abstract

The spatial clustering properties of H i 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 H i galaxies will be restricted to the local Universe for the foreseeable future, even with the deepest H i surveys. Here, we discuss how clustering studies of the H i 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 H i 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 H i within dark matter haloes at z > rsim 0.5 where individual galaxies cannot be detected. In addition to the HOD of H i, the amplitude of the 21-cm power spectrum would also yield estimates of the cosmic H i content at epochs between the local Universe and redshifts probed by damped Lyα absorbers. © 2010 The Authors. Journal compilation © 2010 RAS.