The contribution of quasars to the ultraviolet extragalactic background

The quasar luminosity function and its evolution, as recently derived from multicolor optical surveys, is used to estimate the integrated UV background from observed QSOs as a function of redshift. We compute the spectrum of the diffuse radiation field taking into account the opacity of H and He associated with intervening Lyα clouds and Lyman-limit absorption systems. We show that J912 increases by a factor of 25-35 between the present epoch and z ≃ 2. The attenuation at 912 Å due to the accumulated absorption is a factor of 4-6 over the redshift range 2 < z < 4: at this epoch, the background flux is found to decrease slowly from a peak value of J912 ≃ 1 - 2 × 10-22 ergs cm-2 s-1 Hz-1 sr-1. The estimated ionizing flux is consistent with 2 σ upper limits to the Gunn-Peterson optical depth at high redshifts only for ΩIGMh503/2 ≲ 0.03. Also, the UV radiation from the observed QSOs falls slightly short of the amount needed to satisfy the "proximity effects." We discuss the redshift evolution of absorption systems in terms of such a QSO-dominated background.