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

1. INTRODUCTION

From the beginnings of space research, attempts were made to measure the cosmic far ultraviolet ( 1000-2000 Å) background. This work was strongly motivated by the hope that in this waveband a true extragalactic flux could be detected and characterized. Theoretical speculation as to possible sources for this radiation was unconstrained by the available data and included such diverse processes as emission from a lukewarm intergalactic medium, emission from hot gas produced in a protogalaxy collapse phase in the early universe, the summed emission from a star formation burst phase in young galaxies, and photons from the electromagnetic decay of real or hypothetical exotic particles that were produced, or may have been produced, in the early universe.

It was the expectation that all of the problems that had bedeviled attempts to measure the cosmic extragalactic flux in the optical band would be overcome: the zodiacal light would be absent because the Sun's radiation falls rapidly in the ultraviolet, the measurements would be made above the atmospheric airglow, and the difficulty of separating the diffuse background radiation from that produced by stars would be reduced or even eliminated since the ultraviolet-producing early stars would be limited to the Galactic plane.

The first twenty years of measurements, carried out by a number of groups in at least five countries, covered the entire far ultraviolet band, but for a number of technical and astrophysical reasons, these efforts were concentrated on the wavelength band from 1300 to 2000 Å. The results obtained indicated that the flux was uniform across the celestial sphere and hence was cosmological in origin. Estimates of the intensity of this flux, however, varied by three orders of magnitude, with no clustering around a mean. For a review of this initial work and a discussion of the reasons for these discrepant results, the reader is referred to Davidsen, Bowyer, and Lampton (1974) and Paresce and Jakobsen (1980). It is clear that a major problem with many of these measurements was the need for an uncertain correction for stellar signals. It is perhaps most useful to state that these initial results provided empirical evidence that measurements of the diffuse far ultraviolet background are intrinsically difficult.

A turning point in the study of this background occurred in 1980, when results were obtained from a far ultraviolet channel of a telescope flown as part of the Apollo-Soyuz mission. This instrument had a relatively large throughput and a sufficiently small field of view that a reasonable attempt could be made to exclude stars from the data set rather than to make a typically large and uncertain correction for their effects. The subset of highest quality data from this experiment exhibited a correlation between intensity and Galactic neutral hydrogen column as derived from 21 cm radio measurements (Paresce, McKee, and Bowyer 1980). Although these results were criticized on a variety of grounds, they were quickly confirmed by data from a large fraction of the sky obtained with an instrument on the D2B satellite. This telescope also had a small field of view and a large throughput (Maucherat-Joubert, Deharveng, and Cruvellier 1980). These results showed that the vast majority of the far ultraviolet flux was connected with processes in our Milky Way Galaxy and was not, in fact, extragalactic in origin. Although both of these data sets were consistent with a small part of this flux being isotropic, there was no way to determine whether this component was due to residual airglow processes at satellite altitudes, or to processes occurring within the Galaxy, or to extragalactic phenomena.

The realization that much, or even all, of the far ultraviolet background was Galactic in origin fundamentally changed the character of research in this field. Subsequently, substantial progress has been made and the results obtained have had an impact on a wide range of astrophysical problems.