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

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1.1 The Cosmic Background Spectrum

The topic is the diffuse cosmic ultraviolet background radiation, which is one among a number of diffuse celestial backgrounds that occur at various frequencies. For an excellent earlier review, see Paresce & Jakobsen (89). Figure 1 provides a key for the introductory discussion of the context within which the ultraviolet background occurs.

Figure 1

Figure 1. Diffuse background radiations extend from the radio (circled number 1) to gamma rays (circled 9). The + sign identifies the general spectral region that is the subject of the present review.

The units of Figure 1 are essentially the same as those used in a similar diagram by Longair (63). Diffuse background radiation units that are used by most observers of the ultraviolet background are photons cm-2 s-1 sr-1 Hz-1; such units will be referred to simply as ``units'' throughout this article. There are those that express surface brightness as nIn, where the units of In are ergs cm-2 s-1 sr-1 Hz-1 (e.g. 94). When multiplied by a (constant) factor of 5 x 107, these are ``units.''

Referring to the circled numbers in Figure 1, the various diffuse backgrounds are as follows:

  1. The continuum radio background (74), which has a spectrum described by Yates & Wielebinski (117). This background has its origin in synchrotron radiation from cosmic-ray electrons that are traversing the magnetic field of the galaxy. The downturn in the spectrum at the lowest frequencies is caused by free-free absorption by the ionized component of the interstellar gas.

  2. The microwave cosmic background radiation, discovered by Penzias & Wilson (95). The spectrum is blackbody emission at 2.736 ± 0.017 K (28; see also 69).

  3. Emission from cold interstellar dust. This has been observed by IRAS as the 100 µm cosmic cirrus (64). The existence of such dust at moderate and high Galactic latitudes will be of great interest in our discussion of the origin of the diffuse ultraviolet background.

  4. The predicted integrated emission from redshifted galaxies; two extreme models are shown.

  5. Emission from hot dust in the solar system. This has been mapped, at 12 µm and 25 µm, using IRAS.

  6. Optical background radiation, which is dominated by zodiacal light: solar photons scattering from interplanetary dust.

  7. This curve is a prediction, by Weymann (115), of the spectrum of optical, ultraviolet, and X-radiation that is expected from a dense ionized intergalactic medium, if such exists, for a certain history of the reheating of that medium (see also 50). The large bump is HI 1216 Å Lyman alpha radiation, redshifted into the visible, while the narrower bump is HeII 304 Å radiation, redshifted to about 1500 Å. Region 7 includes the spectral region of the present review; an enlargement, for more detailed description of our context, appears in Figure 2.

  8. The X-ray background, discovered by Giacconi et al (26), and reviewed by Boldt (7). While the spectrum is exquisitely free-free in shape, the perfect blackbody spectrum of the microwave background (Region 2) almost eliminates the possibility that these X-rays are, in fact, radiation from a very hot intergalactic medium (99); the origin therefore remains a mystery.

  9. The gamma ray background. The diffuse Galactic gamma-ray emission is reviewed by Bloemen (4).

Figure 2

Figure 2. (A magnified view of part of Figure 1.) The Apollo 17 point is the cosmic background measurement of Henry et al (37). The solid curve is one prediction, by Weymann (115), of emission from hot ionized intergalactic matter. The hydrogen ionization edge at 912 Å is indicated by a vertical hatched line; shortward of this wavelength, the interstellar medium is almost opaque (78), as illustrated in the attenuation (dashed line) of the Weymann model.

A magnified view, in the same units, of the most immediately relevant portion of the universal cosmic background spectrum appears in Figure 2. The theoretical spectrum of Weymann is repeated. The hatched region at highest energies is the cosmic X-ray background that has been reviewed by Boldt (7), while the observations of Henry et al (38), Davidsen et al (19), and those reviewed by Silk (104) show the sharp rise that is the low-energy diffuse X-ray background. This subject was reviewed recently by McCammon & Sanders (71). The mechanism is emission from fairly local interstellar gas.

There is a ``censored'' region, from 912 to about 44 Å, over which we cannot observe the true cosmic diffuse background because of the very high opacity of the local interstellar medium (78). This high opacity is caused by photoionization of the interstellar gas. This ``censored'' region is shown by a heavy bar near the abscissa in Figure 1; it is bounded by a vertical bar at 912 Å in Figure 2. The extreme efficacy that is expected of the censorship can be seen by considering the dashed line in Figure 2 which shows the expected attenuation by the interstellar gas toward the Galactic pole of Weymann's predicted spectrum. A true diffuse background probably does occur in this energy range, arising from emission from the hottest component of the interstellar medium itself. A comprehensive summary of the observations is provided by Labov, Martin & Bowyer (59), and reviews of the relevant astronomy, which is the local interstellar medium, are given by Holzer (45) and Cox & Reynolds (18). Hence the subject is not discussed further here.

The spectral region to be discussed in what follows ranges from the end of the visible spectrum around 4000 Å, down to 912 Å; with greatest emphasis on the still more limited spectral region between 2500 and 912 Å. How could such a very small wavelength range, which occupies only a minute segment in Figure 2, deserve the attention it receives in this volume of the Annual Review of Astronomy and Astrophysics?

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