On the importance of local sources of radiation for quasar absorption line systems

A generic assumption of ionization models of quasar absorption systems is that radiation from local sources is negligible compared with the cosmological background. We test this assumption and find that it is unlikely to hold for absorbers as rare as H (I) Lyman limit systems. Assuming that the absorption systems are gas clouds centered on sources of radiation, we derive analytic estimates for the cross section-weighted moments of the flux seen by the absorbers, of the impact parameter, and of the luminosity of the central source. In addition, we compute the corresponding medians numerically. For the one class of absorbers for which the flux has been measured, damped Ly alpha systems at z approximate to 3, our prediction is in excellent agreement with the observations if we assume that the absorption arises in clouds centered on Lyman break galaxies. Finally, we show that consistency between observations of the UV background, the UV luminosity density from galaxies, and the number density of Lyman limit systems would require escape fractions of order 10%.