A Method to Measure the Mass of Damped Ly-alpha Absorber Host Galaxies Using Fluctuations in 21cm Emission

Abstract

Observations of damped Ly-alpha absorbers (DLA) indicate that the fraction of hydrogen in its neutral form (HI) is significant by mass at all redshifts. This gas represents the reservoir of material that is available for star formation at late times. As a result, observational identification of the systems in which this neutral hydrogen resides is an important missing ingredient in models of galaxy formation. Precise identification of DLA host mass via traditional clustering studies is not practical owing to the small numbers of known systems being spread across sparsely distributed sight lines. However following the completion of reionization, 21cm surface brightness fluctuations will be dominated by neutral hydrogen in DLAs. Observations of these fluctuations will measure the combined clustering signal from all DLAs within a large volume. We show that measurement of the spherically averaged power-spectrum of 21cm intensity fluctuations due to DLAs could be used to measure the galaxy bias for DLA host galaxies when combined with an independent measurement of the cosmological HI mass density from quasar absorption studies. Utilising this technique, the low frequency arrays now under construction could measure the characteristic DLA host mass with a statistical precision as low as 0.3 dex at z~4. In addition, high signal-to-noise observations of the peculiar-motion induced anisotropy of the power-spectrum would facilitate measurement of both the DLA host mass and the cosmic HI density directly from 21cm fluctuations. By exploiting this anisotropy, a second generation of low frequency arrays with an order of magnitude increase in collecting area could measure the values of cosmic HI density and DLA host mass, with uncertainties of a few percent and a few tens of percent respectively.