Redshifted 21 cm emission from the pre-reionization era.: II.: HII regions around individual quasars

We use cosmological simulations of reionization to predict the effect of large H II regions around individual high-redshift quasars on the possible signal from the redshifted 21 cm line of neutral hydrogen in the pre-reionization era. We show that these H II regions appear as "spectral dips'' in frequency space with equivalent widths in excess of 3 mK MHz or depths in excess of about 1.5 mK, and that they are by far the most prominent cosmological signals in the redshifted H I distribution. These spectral dips are expected to be present in almost every line of sight. If the spectral dips of a large enough sample of H II regions are well resolved in frequency space, then the distribution of line depth and equivalent width in frequency with a known observing beam size can be used to infer the H II region size distribution and the mean difference in neutral hydrogen density between the H II regions (which may contain self-shielded neutral gas clumps) and the surrounding medium, providing a powerful test for models of reionization.