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

We discuss the reionization of the intergalactic medium (IGM) by quasars at high redshift. We compute the integrated UV background from observed QSOs, taking into account the hydrogen opacity associated with intervening Lyα clouds and Lyman limit systems. We note that the published data appear to indicate a significant underdensity of absorption systems in the Lyα forest with column densities N HI >10 15 cm −2 . This deficit, if real, would result in a reduction of the opacity of the universe at high redshift by a factor of 1.5-3 relative to previous estimates