  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.
