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An upper limit to the importance of local sources of radiation compared tothe cosmic background in cosmological absorption systems is derived, as asimple consequence of the conservation of surface brightness. The limit dependsonly on the rate of incidence of the absorbers and the mean free path of theradiation. It is found that, on average, the ionizing radiation intensity fromlocal sources in Lyman limit systems at z>2 must be less than half of theintensity of the cosmic background. In absorbers with column densities muchlower than Lyman limit systems, the local source contribution must benegligible. The limit on the ratio of local source to background intensities isthen applied to the class of damped Lya absorption systems with detectableexcited CII lines. A cooling rate of the gas in these systems has been measuredby Wolfe et al., who assumed that the balancing heating source is photoelectricheating on dust by light at ~ 1500 A . The intensity from local star formationat this wavelength in this class of damped Lya systems is found to be at most ~3 times the background intensity. If the heating source is indeed photoelectricheating of dust, the background created by sources associated with damped Lyasystems can then be estimated from the average cooling rates measured in theabsorbers. Current results yield a background intensity higher than previousestimates based on observed galaxy and quasar luminosity functions, althoughwith a large uncertainty. The possibility of other sources of heating, such asshock-heating in a turbulent medium, should be explored.Comment: submitted to ApJ

On the Importance of Local Sources of Radiation in Cosmological Absorption Systems