Annu. Rev. Astron. Astrophys. 1991. 29: 59-88 Copyright © 1991 by Annual Reviews. All rights reserved

6. THE EXTRAGALACTIC BACKGROUND AND RELATED EXOTICA

The most commonly cited argument that the diffuse far ultraviolet background is in part, or entirely, extragalactic is that this flux is uniform over the celestial sphere, at least at high Galactic latitudes. While it is now clear that the data supporting this argument are deficient and most of the flux is due to processes in the Galaxy, it is also clear that some minimum flux (typically a few hundred CUs) is present in all view directions at high Galactic latitudes. Further, as summarized in Table 1, the intensity of this flux is still finite when extrapolated to zero neutral Galactic hydrogen column. It may well be that this flux is uniform across the sky and the scatter in the data reported in Table 1 only represents experimental uncertainty. Even if this is true, however, it is by no means established that this flux is extragalactic since it could be the product of geocoronal, solar system, or Galactic effects. Indeed, Hurwitz, Bowyer, and Martin (1991) and Martin, Hurwitz, and Bowyer (1990) have provided evidence discussed earlier in this review that the majority of this ``uniform'' flux is Galactic in origin.

Despite these uncertainties, it has been established that the far ultraviolet background does convey information on a variety of topics that might be classified as ``cosmological'' in character. I review some of these topics in the following.