Quasar ionization of Lyman-alpha clouds - The proximity effect, a probe of the ultraviolet background at high redshift

We study the distribution of lines in the Lyα forests in quasar spectra, using spectral data from 19 quasars with emission-line redshifts 1. 7 nu_ = -21. 0+/-0. 5, where Jnu_ is the Lyman-limit intensity in units ergs cm^-2^ s^-1^ Hz^-1^ sr^-1^. This is larger than the integrated UV emission of observed quasars at z > 3, so there must exist appreciable sources of ionizing photons other than quasars at high z, or else many high-z quasars must be obscured by intervening dust. The redshift evolution index for the general Lyα cloud population, γ, is found to be systematically larger than otherwise when the proximity effect is correctly taken into account. In our sample, we find γ = 2. 36+/- 0. 40. If our conclusions are confirmed by studies using a larger statistical base, then the "proximity effect" can used as a powerful tool to measure locally the ionizing flux emitted by high-redshift objects. It could be used to check if BL Lac objects emit strongly beamed radiation as predicted by models, to study stochastic variations in OVV quasars, and to measure quasar flux amplification by gravitational lenses.