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

1.2 The Importance of the Diffuse Ultraviolet Background

There are two reasons why diffuse emission in this narrow band is of great importance. The first is that a host of disparate, quite unconnected emission sources are either known to exist, or might exist, which could contribute to this diffuse background. Measurement of these emissions would greatly improve our understanding in many areas of astronomy and astrophysics.

The second reason of even greater potential importance is that the sky may be truly outstandingly black in the far ultraviolet, offering a ``dark site'' that is unprecedented in astronomy. Figure 3 is from a discussion of this by O'Connell (85). From 2500 Å to longer wavelengths O'Connell's figure shows that the cosmic diffuse background is dominated by zodiacal light. This is why we concentrate here on the region from 2500 Å down to 912 Å; nevertheless, the zodiacal light is of considerable interest in its own right, and so the state of these observations also will be mentioned in what follows.

Figure 3. Estimated spectral energy distribution of the night-sky background near the zenith at an excellent ground-based site on a moonless night and in a direction typical of extragalactic pointings in space. The curves are plotted in monochromatic magnitude units. The wavelength at which the diffuse Galactic light (DGL) and zodiacal (ZODI) contributions to the space background are comparable for this pointing direction is indicated, and the arrows indicate the regions of dominance of one or the other. Effects of individual strong skyglow emission lines are not included, but the combined effect of OH emission bands on the ground is evident for > 7000 Å. [Figure and adapted caption from O'Connell (85), with permission.]

Shortward of 2500 Å, O'Connell shows ``diffuse galactic light'' as the source of the cosmic ultraviolet background, with an average level of emission of 25.5, in his units, which corresponds to about 1000 units at 2000 Å. But there are very great disagreements among various observers as to what the actual brightness of the sky is at these wavelengths at moderate and high Galactic latitudes. Most observers do agree that there are many places at high Galactic latitudes where the diffuse background is < 300 units. The details of the various observations, and their disagreements, form the bulk of the present review, but the point to be made here is that O'Connell is probably more than a magnitude conservative in Figure 3, in the sense that the sky is probably even blacker at moderate and high latitudes, in the ultraviolet, than O'Connell suggests.

Why is this darkness important? First, examine the 4000 to 6000 Å spectral range in Figure 3. In this range, the improvement in background that can be expected for Space Telescope over the background experienced by ground-based observatories is only about one magnitude. Space Telescope will make its greatest improvement in detecting faint point sources (once the replacement wide-field/planetary camera is installed), by concentrating the light of point sources and thus increasing their detectability against the zodiacal light background. But in the ultraviolet, as O'Connell shows, the background itself is reduced by perhaps four magnitudes compared with the visible!

This makes the degradation of the usefulness of the European Faint Object Camera on the Hubble Space Telescope tragic, with no replacement planned, but it also makes clear the enormous promise for the future if an intensely black ultraviolet sky at moderate and high Galactic latitudes, in fact, exists.

There is also the question of detection of extended objects, and in particular, objects of very low surface brightness. O'Connell concludes, ``taking into account the UV/V energy distributions of potential targets, we find that in certain favorable circumstances UV photometry may permit the detection of regions with equivalent V band surface brightnesses as low as 35 mag arsec^-2, or over 100,000 times fainter than the ground-based night sky. We consider applications of UV surface photometry to the study of circumgalactic regions, dwarf galaxies, low-surface-brightness spirals, and the detection of primeval galaxies.... .''

Of course even the darkest sky is of no use, unless there are sources of interest radiating in the spectral band where the dark sky occurs, and O'Connell considers that point, as may be seen from the above quote. Some very faint galaxies are blue (12, 112). Also, our knowledge of galaxies is heavily biased by the sky background (20). Low-surface-brightness galaxies have been discovered (9, 96). Now, our spectral region includes the resonance line of the most abundant atom in the universe, hydrogen. This line, Lyman , is at 1216 Å. Observations right at 1216 Å are severely impeded by local (solar-system) sources of Lyman (see below), but for moderate redshifts, Lyman that is emitted by galaxies, by the intergalactic medium if any, or by as yet undiscovered objects lies in ``the black hole,'' of the intensely dark ultraviolet sky. But does this darkness exist? What now is necessary is a discussion of the status of the relevant observations.

The field of diffuse ultraviolet background radiation is controversial for two reasons: first, the observations are difficult, for reasons to be explained; and second, each set of observations is necessarily the result of a space experiment, which is costly both in dollars and in human investment, and there is then even more intensity than customary in defending data, some of which must be incorrect because different data sets are contradictory. In this regard, consider the comment by Bondi (8): ``while in observational work it is unfortunately considered somewhat impolite for one observer to criticize the observations and immediate inferences of other observers, similar criticism between theorists is luckily considered perfectly natural''. In what follows it will be necessary to discuss and criticize some of the contradictory data sets, and we do so in the spirit of Bondi.