Many astronomical sources are capable of emitting UV light with photon
energies in
the range h
10-20 eV through various
thermal and non-thermal emission processes.
The diffuse cosmic background radiation at ultraviolet
1000 - 2500 Å wavelengths
is therefore of interest for a wide variety of topics. These include the
study of the
properties of interplanetary and interstellar dust grains, thermal line
emission from the
interstellar and intergalactic gas, the integrated light of galaxies,
and radiative decay of exotic cosmological particles.
Reviews of the topic of the diffuse UV background have been given by
Davidsen, Bowyer &
Lampton (1974),
Paresce & Jakobsen (1980),
and most recently by
Bowyer (1991) and
Henry (1991).
Not least thanks to the experimental efforts of the Berkeley
group, the rather confusing observational situation surrounding the
far-ultraviolet
background has lately become much clearer. There is today good evidence
that the
diffuse ultraviolet background is largely dominated by dust scattering
and other interstellar emission processes occurring within the Milky Way
(Martin & Bowyer 1990;
Martin, Hurwitz & Bowyer
1990;
Hurwitz, Bowyer & Martin
1991).
Although there
remains some disagreement as to the relative strengths of the various
interstellar
contributors (cf. Henry, this conference), there is general consensus
that the intensity of
any quasi-isotropic and therefore possibly extragalactic component to
the background
in the 1300 - 2000 Å
range is approximately
I
100 photons s-1
cm-2 sr-1 Å-1,
give or take a factor of ~ 2-3.
There are at present three candidate extragalactic sources of UV radiation that could conceivably produce a diffuse background flux of this intensity: i) diffuse thermal emission from the intergalactic medium; ii) the integrated UV light of galaxies and quasars; and iii) radiative decay of massive neutrinos. In what follows, each of these emission sources is discussed in turn.