Abstract
We discuss the evolutions of the intensity and spectrum of the UV background radiation (UVB) in flat and open cold dark-matter universes. We consider two kinds of UV sources: one is QSOs, the other is collapsing objects, which liberate their binding energy radiatively. In the latter sources, we include the line emission corresponding to collisional excitation cooling of He II as well as free-free emission. We find that UV photons emitted from collapsing objects contribute to UVB as much as those from QSOs, and that the UVB spectrum becomes softer than that in the case in which only QSOs are taken into account. We also calculate the Gunn-Peterson optical depths assuming the ionization equilibrium, and compare them with observations.
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