On the photoionization of the intergalactic medium by quasars at high redshift

Abstract

The reionization of the intergalactic medium (IGM) by quasars at high redshift is discussed. The integrated UV background is computed from observed QSOs, with the hydrogen opacity associated with intervening Ly-alpha clouds and Lyman limit systems taken into account. The degree of attenuation of the UV background by intervening absorption systems is made uncertain by the poorly known column density distribution of the absorbers. It is argued that the IGM may be highly clumped, and two models for the dynamics of the discrete absorbers are considered. There are large uncertainties in the estimated values of the Gunn-Peterson optical depth. The existence and amount of a decline in the comoving space density of low-luminosity QSOs at z of 3-5 is also highly uncertain. It is concluded that in several of the models studied, the observed QSOs alone can provide the number of ionizing photons required at high redshift to satisfy the observational constraints.