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

An upper limit to the importance of local sources of radiation compared to the cosmic background in cosmological absorption systems is derived as a simple consequence of the conservation of surface brightness. The limit depends only on the rate of incidence of the absorbers and the mean free path of the radiation. It is found that on average, the ionizing radiation intensity from local sources in Lyman limit systems at z > 2 must be less than half of the intensity of the cosmic background. In absorbers with column densities much lower than Lyman limit systems, the local source contribution must be negligible. The limit on the ratio of local source to background intensities is then applied to the class of damped Lyα absorption systems with detectable excited C II lines. A cooling rate of the gas in these systems has been measured by Wolfe et al., who assumed that the balancing heating source is photoelectric heating of dust by light at ~1500 Å. The intensity from local star formation at this wavelength in this class of damped Lyα systems is found to be at most ~3 times the background intensity. If the heating source is indeed photoelectric heating of dust, the background created by sources associated with damped Lyα systems can then be estimated from the average cooling rates measured in the absorbers. Current results yield a background intensity higher than previous estimates based on observed galaxy and quasar luminosity functions, although with a large uncertainty. The possibility of other sources of heating, such as shock-heating in a turbulent medium, should be explored.