  Deep surveys have recently discovered galaxies at the tail end of the epoch
of reionization. In the near future, these discoveries will be complemented by
a new generation of low-frequency radio observatories that will map the
distribution of neutral hydrogen in the intergalactic medium through its
redshifted 21cm emission. In this paper we calculate the expected
cross-correlation between the distribution of galaxies and intergalactic 21cm
emission at high redshifts. We demonstrate using a simple model that overdense
regions are expected to be ionized early as a result of their biased galaxy
formation. This early phase leads to an anti-correlation between the 21cm
emission and the overdensities in galaxies, matter, and neutral hydrogen.
Existing Ly-alpha surveys probe galaxies that are highly clustered in overdense
regions. By comparing 21cm emission from regions near observed galaxies to
those away from observed galaxies, future observations will be able to test
this generic prediction and calibrate the ionizing luminosity of high-redshift
galaxies.
