Annu. Rev. Astron. Astrophys. 1991. 29: 89-127 Copyright © 1991 by Annual Reviews. All rights reserved

6.2 Is There Light Scattered From Dust?

As described above, many data sets, if considered in isolation, suggest that the light of the OB stars of the Galactic plane scattering off of dust located at high Galactic latitudes has been seen. However, many of these data sets are contradictory.

Among all of the data sets in the 1216 to 1800 Å range the one subset that seem to be decisive is the large number of 300 unit observations of Paresce et al (91). [Henry (32) has pointed to a good reason that these observations should be considered to read 400 units, not 300 units.] It seems certain that these observations are correct, at least if they are treated as upper limits. These numerous upper limits are, in my opinion, the best evidence that there is no significant amount of light scattered from dust at moderate and high latitudes. This is not an unreasonable conclusion, because the IRAS observations of strong cosmic cirrus at 100 µm mean that the dust must be strongly absorbing at some wavelengths where there is significant energy input from stars in the Galactic plane.

I cannot explain the high intensities at moderate latitudes that appear in Figures 16 and 17, but the Paresce et al (91) data suggest that those intensities may be incorrect. For the future, a well-calibrated deep ultraviolet image of the optically reflecting dust cloud at b = +40° (100) would be sufficient to prove this view right or wrong.

The above remarks apply only to |b| > 30°, but there is some evidence that even at the lowest latitudes there is very little light scattered from dust in that half of the Galactic (or rather, Gould) plane that is relatively free of stars that are bright in the ultraviolet (Figure 7). In the brighter parts of the Gould belt, a fairly bright diffuse background may be present. Of course at some level, it is inevitable that light scattered from dust must be present at high latitudes, but the claim here is that the amount is small compared with 400 units.