A final more speculative potential source of diffuse extragalactic UV
emission is
the radiative decay of exotic particles of cosmological origin. Since
Big Bang theory
predicts the existence of a cosmological "sea" of neutrinos having a
particle density
similar to the photon density of the 2.7 K microwave background
radiation, massive
neutrinos and similar "inos" of various flavors have long been
considered as candidate
sources for missing matter. In particular, any type of exotic particle
having a present day cosmological particle density of
n0
100 cm-3 will be
capable of closing the Universe if its mass is of order
m
100h2 eV.
In some theories, such massive exotic particles are not stable, but decay into lighter particles under the emission of photons. It is easy to show that the energy of such decay photons is given by
| (17) |
where mH and mL are the masses of
the heavy and light particles, and the last
approximation is valid in the limit where mH >>
mL. From this equation it follows that any
particle massive enough to provide closure density will have emit its
decay radiation at
E
50 h2 eV,
i.e. at ionizing extreme-UV wavelengths.
Depending on the rate of decay of the particles in question, the accumulated
redshift-smeared emission from such particles could give rise to observable UV
background radiation. Conversely, the UV background observations
severely constrain the
exponential decay time of any radiatively decaying cosmologically
produced particle with a mass in the range
m
10 - 100 eV to be much
larger than the age of the
Universe, or 
> 1023 s
(Kimble, Bowyer &
Jakobsen 1981;
Overduin, Wesson &
Bowyer 1993).
The intensity and spectral shape of the decay background is easily calculated from equation (1) through insertion of the appropriate line emissivity
| (18) |
where 
n /
is the particle density decay rate. This leads to
| (19) |
where 
0 and
l are the observed
and decay line wavelengths. The redshifted decay
spectrum displays a characteristic jump at
0 =
l and drops
steeply toward the red
as I
-2.5
(
1).
Following the suggestions of
Cowsik & McClelland (1972)
and De Rújula &
Glashow (1980),
much attention has in recent years focussed on the concept of decaying massive
neutrinos as possible carriers of the missing mass. The astrophysical
consequences of this idea have been investigated in some detail by
Sciama (1990)
who in a series of
papers has argued that through suitable tuning of the parameters,
massive decaying
neutrinos are not only capable of explaining the missing mass problem,
but also provide
a convenient and omnipresent in situ source of ionizing radiation
that is capable of
explaining the ionization structure of both the interstellar medium of
the galaxy
(Sciama 1993)
and the intergalactic Lyman forest clouds discussed in
Section 2.1
(Sciama 1991).
In order to accomplish all this, the neutrino properties need to be
rather tightly
constrained. Sciama's hypothetical neutrinos have a mass of
m
28 eV and decay under
emission of photons at a wavelength of
890 Å just below the
Lyman limit. The neutrino exponential decay time is
1 - 2
1023 s.
Figure 6 shows the resulting redshift smeared
far-UV background calculated for
these parameters. Although the edge of the decay spectrum is
conveniently hidden by
interstellar neutral hydrogen absorption, and therefore un-observable,
both the absolute
intensity (I
(100 - 600)
h-1 photons s-1 cm-2
sr-1 Å-1) and the steep blue color
of the decay spectrum are at best barely consistent with the existing
observations of the UV background (cf. figure 9 of
Martin et al. 1991),
but can probably not be definitively
excluded at this point given the uncertainties in the data
(Overduin et al. 1993).
|
Figure 6. The redshift-smeared UV
background predicted for the decaying neutrino model of
Sciama (1990).
The step in the spectrum at the decay wavelength of
|
Other and more persuasive observations that point against the Sciama model have, however, been reported by Davidsen et al. (1991) and Dettmar & Schultz (1992) (and countered in Sciama 1993).