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

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7. CONCLUSIONS

In the past ten years, remarkable progress has been made in our understanding of the diffuse far ultraviolet background. In the beginning of the 1980s, the consensus was that this background was primarily extragalactic in origin. Estimates of its intensity ranged over three orders of magnitude. Its spectrum was unknown. We now know that this flux is primarily Galactic in origin, but a small extragalactic component has probably been detected.

A number of processes that contribute to this background have been identified. The primary source of the background is scattering by dust. This dust appears to have a low albedo and to scatter isotropically and hence is different from the dust that produces scattering in the visible spectrum. There is preliminary evidence that this dust is present in all view directions in the Galaxy.

Emission from hot (~ 105 K) gas has been detected. An analysis of this radiation establishes that the emitting gas is well above the Galactic plane and thus resolves a long-standing controversy regarding the origin of the interstellar high ionization lines seen in absorption in studies carried out with the IUE satellite. The so-called Galactic fountain model for the origin of this gas is consistent with the data.

Two-photon emission from recombining ionized hydrogen has been recognized as a component of the far ultraviolet background. Molecular hydrogen fluorescence was found to be present in low density molecular clouds. The data indicate that this emission is primarily produced from clumped portions within the clouds.

fig10
Figure 10. Summary data on the cosmic far ultraviolet background. The data from 912 to ~ 1200 Å are from Holberg (1986) and are upper limits to the flux from a high Galactic latitude view direction. Two data sets are shown for the 1400 to 1850 Å band. The upper line is from Hurwitz et al. (1991) and shows typical data obtained at a high Galactic latitude; the CIV 1550 Å line is clearly evident in emission and the 1663 Å line of OIII] is also apparent, though at lower signal-to-noise ratio.

In Figure 10, I provide examples of the best available data on the diffuse far ultraviolet background. The data from 912 to ~ 1200 Å are from Holberg (1986) and are upper limits to the background from a high Galactic latitude view direction. Two data sets are shown for the 1400-1850 Å band. The upper line is from Hurwitz, Bowyer, and Martin (1991) and shows data obtained at a low Galactic latitude. These data are typical of what is observed in view directions with an optical depth of taudust geq 1. Molecular hydrogen fluorescence is evident as an additional component at wavelengths from 1550 to 1650 Å. The lower line is from Martin and Bowyer (1990) and shows data obtained at a high Galactic latitude and low total neutral hydrogen column. The CIV 1550 Å line is clearly evident in emission, and the 1663 Å line of forbidden OIII is also apparent, though at lower signal-to-noise ratios.

The only extragalactic component that appears to have been detected is the summed far ultraviolet emission of all galaxies. If the reported flux is taken only as an upper limit to the true flux from galaxies, significant limits can be placed on the star formation rate in the universe for the past one-third of a Hubble time.

Table 3. Components of the Diffuse Cosmic Far Ultraviolet Background with Approximate Intensities a
Table 3

A summary of the major components of the cosmic far ultraviolet background is presented in Table 3. It is clear that studies of the diffuse ultraviolet background have at long last begun to bear fruit. The results obtained have not only delineated the origins of this flux, they have also enriched our understanding of diverse areas of astrophysical interest.

ACKNOWLEDGMENTS

I would like to thank Mark Hurwitz for useful discussions and for assistance in assembling the tabular data used in this review. This work was supported by NASA grant NGR-05-003-450, which is administered by the Space Sciences Laboratory of the University of California at Berkeley.

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