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.