In his review of the diffuse background Henry (1991) consistently took an extremely skeptical attitude toward claims of the detection of diffuse ultraviolet radiation, and particularly toward works that claimed to understand the physical source of the radiation absent a spectrum. In particular, it was only in the case of the Voyager observation of extended diffuse emission in Ophiuchus by Holberg (1990) that Henry felt that there was a very strong case for the assertion that ultraviolet starlight scattered from dust had been detected. There is now a second very strong case: Murthy, Henry, and Holberg (1993) have detected extremely strong scattered starlight in the direction of the Coalsack nebula (see Figure 7). Detailed modeling shows that this is not light backscattered from the Coalsack, but rather is the forward-scattered light of three very bright ultraviolet stars near the Coalsack. The spectral dependence of this diffuse emission is (as can be seen by our model fit) exactly that of the illuminating early B stars. Unless g is varying with wavelength in such a way as to fortuitously exactly cancel changes in a, we can conclude that the albedo of the grains is as high at 1000 Å as it is at 1350 Å. In fact, exactly the same phenomenon can be seen in Figure 2 of Holberg (1990), where the geometry of the dust relative to the source star is not so clear, but is unlikely to be the same as for the Coalsack observation.
This is an important result, highly relevant to the question of what is
happening at
high galactic latitudes. Voyager, with its low sensitivity longward of
Lyman 
, could not
be expected to detect the background that appears in
Figure 1. But it should definitely
detect 300 units in the range 1000 Å to 1100 Å if it is there,
and furthermore, the data
of Figure 7 suggest that if what is being seen
at high latitudes at longer wavelengths
by many independent observers is starlight scattered from dust, the
spectrum should
continue strongly down to 912 Å. Voyager shows that it does not.
We have seen, in Figure 1, the
Voyager upper limit of 100 units at 1100 Å. The
extragalactic radiation field at slightly shorter wavelengths has been
measured by
Kulkarni and Fall (1993)
by applying the proximity effect to Lyman
forest lines in the spectra
of nearby quasars. They find a intensity of one unit, well below the
Voyager upper limit at 1100 Å.
|
Figure 7. Spectrum of the Coalsack nebula
as observed by
Murthy, Henry, and
Holberg 1993.
This is the brightest cosmic diffuse ultraviolet radiation ever
reported in the night
sky. The radiation is the forward-scattered light of three extremely
bright ultraviolet-emitting
stars, |
Holberg (1986,
1990) and
Murthy, Henry, and
Holberg (1991)
present the evidence
for the validity of the Voyager upper limit. There is a great deal more
that can be done
with the Voyager archive, and Murthy, Henry, Hall, and Holberg have been
funded in
an archival research program to carry out this project, which is under
way. In Figure 12 of his review
Henry (1991)
indicated that every Voyager diffuse background observation
above 20° latitude was only an upper limit. We have subsequently
learned (Holberg,
private communication) that the Voyager targets selected for
analysis included only
those that showed no evidence of a signal. That does not change any
conclusions:
there is still the same considerable number of locations at moderate and
high galactic
latitudes that show only an upper limit (which does not occur at
wavelengths longward
of Lyman ,
Figure 1), and those locations where
a signal is present may all contain
point sources (the overwhelming majority of Voyager pointings were toward known
point sources). The new situation does leave open the possibility,
however, that diffuse
emission might still be detected at high latitudes near 1100 Å
using Voyager. Indeed,
if g and a are both large, we would predict a significant
signal at high latitudes near
the location of bright ultraviolet stars (see
Figure 6).
Figure 8 shows the Voyager upper limits of
Holberg (1990) and of
Murthy, Henry, and
Holberg (1991)
superposed on a map of the expected scattered light above b = 40°
predicted using the model of
Onaka and Kodaira
(1991).
We used 
= 0.4 csc b in
making this plot, with the albedo taken as 0.65 and g = 0.9
(northern hemisphere)
and g = 0.8 (southern hemisphere), to illustrate the model. We
have evaluated the
reduced 
2 for these data
(treated as detections, each with a standard deviation of
100 units) against this model as a function of a and g. A
plot of the reduced 2 appears
in Figure 9. An albedo of 0.65 is seen to
require that g > 0.8. A sufficiently low albedo
will also explain the data.
|
Figure 8. The shading shows the predicted
scattered light as a function of galactic longitude and latitude as
predicted using the model of
Onaka and Kodaira
(1991).
We have used albedo a = 0.65, and
|
|
Figure 9. Reduced
|
The new work already done on Voyager data that bears most directly on
the present
discussion is a new observation by Murthy, Henry, and Holberg of the
extended dust
patch at high galactic latitudes that was discovered by
Sandage (1976).
Sandage's
Plate 1 shows very clear evidence for dust at b = +38°. Our
Voyager observation shows
nothing but an upper limit of 100 units, and our measurement is so clean
that we
have included it as part of our data-reduction template for "no
astrophysical signal"
(Murthy, Im, Henry,
and Holberg 1993).
The importance of this observation is that
Sandage deduces that AV = 0.3 mag, from the 21 cm observation
of
Heiles (1975).
This translates into an optical depth
at 1100 Å of 1.0 mag, using
the E1100-V / EB-V of
York et al. (1973).
Use of the model of Onaka and Kodaira for Sandage's location and
value of t shows that for an albedo of 0.65 we require g > 0.9 to
explain our Voyager
result. (Another possible explanation would be a low albedo for the
grains: this of
course would also suggest that the high galactic latitude signal at
longer wavelengths is
extragalactic.) The Sandage region was also scanned at longer
ultraviolet wavelengths
by Murthy et al.
(1989,
1990),
and also by
Martin, Hurwitz, and
Bowyer (1990).
No enhancement of background as the line of sight passed over the
Sandage region was noted by any of the various UVX spectrometers.
Cru, and