The three known potential sources of extragalactic UV background radiation have been discussed with an emphasis on evaluating their possible contributions to a nominal observed extragalactic UV background of intensity I ~ 100 photons s-1 cm-2 sr-1 Å-1.
The most firm conclusions are possible in the case of the integrated UV light of galaxies. In particular, direct galaxy counts in the UV down to a limiting magnitude of m 18.5 have already "resolved" the background to an intensity of I 30 photons s-1 cm-2 sr-1 Å-1, which extrapolated to fainter magnitudes by means of galaxy evolution models suggests that galaxies are probably the all-dominant source of extragalactic UV background radiation. There is also evidence that the UV background exhibits the small scale fluctuations expected from the accumulated light of galaxies.
The conclusion that thermal Ly and HeII 304 Å emission from the intergalactic medium, on the other hand, is a marginal contributor to the UV background is almost nearly as compelling, and is reached by drawing on results obtained through the study of quasar absorption lines. The intensity of the metagalactic ionizing background at high redshifts inferred from the so-called "proximity effect" displayed by the Lyman forest absorption lines suggests that the redshift-smeared backgrounds due to Ly and HeII 304Å recombination radiation from photoionized intergalactic gas are to be found at intensities far below current observational limits on the extragalactic background flux. The results of the Gunn-Peterson test at low redshifts derived from UV quasar spectra obtained with IUE constrain the intensity of redshifted collisionally excited Ly emission from a shock-heated IGM component to an equally low intensity. The possibility that the IGM has gone through a phase of intense collisionally excited HeII 304Å emission at z 3 - 5 cannot be completely ruled out until the HeII equivalent of the Gunn-Peterson test is carried out with HST or Lyman/FUSE. However, the statistics of quasar absorption lines imply that any far-UV background component stemming from z 3 will remain effectively hidden from view because of strong accumulated Lyman continuum absorption in the Lyman forest and Lyman limit types of quasar absorption systems.
The last source of extragalactic UV radiation considered, namely the decay from exotic cosmological particles, is by its nature more speculative and therefore less easy to rule out definitively. Nonetheless, in its latest incarnation as proposed by Sciama, the redshift smeared background predicted from this source has an intensity and spectral shape that strains the limits of existing observations.