Cosmological H II regions and the photoionization of the intergalactic medium

The generalization of the classical H II region problem to the case of a point source of ionizing radiation in a cosmologically expanding gas in a Friedmann-Robertson-Walker universe is summarized. An application of this work is described in which is tested the hypothesis that quasars photoionize the IGM to the degree implied by the well-known absence of a Gunn-Peterson effect. Recent studies of faint quasars at high redshift, which suggest a decline in the number density of quasars for z greater than three, imply that the H II regions of high-redshift quasars cannot overlap early enough to satisfy the Gunn-Peterson test. This suggests that either the observations are failing to detect the true number density of high-redshift quasars or else something else must ionize the IGM at high redshift. 8 references