The correlation between the distribution of galaxies and 21-cm emission at high redshifts

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 21-cm emission. In this paper we calculate the expected cross-correlation between the distribution of galaxies and intergalactic 21-cm 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 anticorrelation between the 21-cm 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 21-cm 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.