  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 Lya absorption systems with detectable
excited CII 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 on dust by light at ~ 1500 A . The intensity from local star formation
at 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 photoelectric
heating of dust, the background created by sources associated with damped Lya
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.
