The Halo Occupation Distribution of HI from 21cm Intensity Mapping at Moderate Redshifts

TL;DR

This paper explores how 21cm intensity mapping can be used to study the distribution of neutral hydrogen in dark matter halos at moderate redshifts, providing a new method beyond direct galaxy detection.

Contribution

It introduces an analytic framework for using 21cm intensity fluctuations to constrain the HI halo occupation distribution at z>0.5, extending clustering studies to higher redshifts.

Findings

21cm power spectrum can probe HI distribution at z>0.5

Sensitivity estimates for SKA Pathfinder to HOD parameters

Potential to measure cosmic HI content at intermediate redshifts

Abstract

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.