We have used the imaging instrumentation on the Dynamics Explorer 1 (DE) satellite to measure the intensity of the diffuse ultraviolet (UV) radiation on two great circles about the sky. We find that the isotropic component of the diffuse UV radiation (possibly of extragalactic origin) has an intensity of 530 ±80 units (a unit is one photon cm−2 s−1 sr−1 A−1) at a wavelength of 150 nm. The galactic component of the diffuse UV radiation has a dependence on galactic latitude, which requires strongly forward-scattering particles if it is produced by dust above the galactic plane.

The UVX experiment has resulted in confirmation that the diffuse ultraviolet background is low in intensity everywhere and that it is fairly uniform in intensity, both spatially and spectrally. There is no clear evidence that any significant portion of the moderate galactic latitude diffuse cosmic background originates in galactic plane starlight scattering from interstellar dust.

NASA's Ultraviolet Experiment (UVX) payload, which flew aboard space shuttle Columbia in January 1986, contained a spectrograph built by the Space Astrophysics Group at the University of California, Berkeley. The wavelength range is 1400–1850 Å with a FWHM resolution of ~15 ± 2 Å. A full description of the instrument can be found in Martin and Bowyer (1984). The instrument was pointed at various regions of the sky for 8 nighttime orbits. Targets spanning a wide range of galactic latitudes and neutral hydrogen column densities were observed.

From visible or infrared brightness observations of the zodiacal light, a large variety of models of the three-dimensional structure of the zodiacal dust cloud has been proposed. To assess the reliability of these models, we must first investigate their fit to a selected set of observational data. The fit is best for bulge models, which have an appreciable density over the solar poles. Next, we check the orbital inclination distributions predicted by the various models. A comparison of these distributions with those of minor bodies in the solar system does not support the preference for bulge models, but instead supports polar hole models with a negligible density over the solar pole. These uncertainties of modelling have to be kept in mind when models are used to derive the brightness contribution of zodiacal light, particularly in the infrared, where the data base is still limited.

A relationship exists between the amount of the ultraviolet (UV) extinction and the relative strength of the very broad structure (VBS) in the 500–600 nm wavelength region (5000–6000 Å) of the interstellar extinction curve. It is based on observations of OB stars distributed along the Milky Way. By examining E(B-V), we found that larger-than-mean values of the UV extinction correspond to larger-than-mean strengths of the VBS and vice versa. This feature is thought to be an emission arising from photoluminescence (PL) of small amorphous carbonaceous grains.

At the boundary of a large expanding shell in Eridanus around l = 187°, b = −50° the morphology observed in the HI emission is well mimicked by the 100 μm surface brightness but with associated structures offset by as much as 0.°5. A point-to-point comparison between I100μm and NHI in filaments of neutral hydrogen and dust (IR cirrus) produces only a weak dependence. However, when I100μm at a cirrus dust peak is compared with NHI at the associated H i peak, a relationship closer to that reported by other workers is found. Preliminary CO observations have set low limits on the molecular gas in these filaments. Since the H i and dust in our region are associated with a large expanding shell (or superbubble), shocks may be responsible for separation of gas and dust. The existence of small-scale structure in both the HI and IR is noted. We conclude that attempts to correlate HI and IR must invoke high-resolution area surveys.

The spatial distribution of interstellar dust is clearly important in understanding not only galactic dynamics but also the processing of the dust and the interstellar medium in general. Probably the best spectral region for investigating interstellar dust is the infrared (IR), where the cool dust is likely to radiate. Indeed, one of the most prominent features of the IRAS sky is the ubiquitous cirrus emission, thought to be due to interstellar dust heated by the interstellar radiation field (ISRF), seen at 60 and 100 μm (Low 1984). However, it is difficult to use the cirrus to probe the dust distribution, both because we have no depth information and also because the cirrus, due to its low temperatures (~20 K), is a probe of high-density dust regions. A far more sensitive search could be made if the dust were hotter, that is, in the presence of a greater ultraviolet (UV) flux. We have made use of this fact to search for dust in the vicinity of hot, bright stars, where even a small amount of dust will dominate the total emission along that line of sight.

We present a new method, based on surface photometry, which allows one to make the spatial resolution of extinction maps of dark clouds as fine as necessary. Applying this technique to the Coalsack (l = 302°, b = 0°), we derived an extinction map and the dust mass MD = (62 ± 25) M⊙. The density was found to vary ρ ~ r−0.8, typical for stable clouds.

Galactic background radiation has been observed in the 78-111 eV Be band using 5000 Å beryllium filters in front of a thin-window proportional counter collimated to a 15° full width at half maximum field of view. Be band data have been analyzed from two sounding rocket flights (Bloch et al. 1986, Juda 1988) that viewed seventeen different directions distributed over the northern galactic hemisphere. In Figure 1 the pointing directions of the two flights are indicated on a map from McCammon et al. (1983) of the 130-188 eV B band count rate.

Ultra-deep CCD surveys to 29 mag from .32 to .9 μ wavelength reveal an isotropic population of very blue galaxies. There are over 150,000 of these objects per square degree per magnitude. Saturation of their number density at 27 mag indicates that most of the optical light from this population has been detected. The resulting extragalactic background radiation from the UV to the near-IR due to this population of objects is shown. Gravitational lens and Lyman-break observations show that the redshift of galaxies fainter than 24 mag is in the range 1–3. Small-scale dark lane structures may be intergalactic dust clouds or tunnels through the luminous galaxy distribution.

We present a review of the presently available observations of the extragalactic background light (EBL) obtained by means of night sky photometry. The EBL is a quantity of great cosmological importance; areas which are directly affected include galaxy formation and evolution, the appropriateness of different cosmological models, and the local luminosity density due to galaxies and other matter in intergalactic space. The basic problem in measuring the EBL is its separation from other, much stronger components of the light of the night sky. None of the different observational techniques have succeeded in providing a generally accepted measurement of the EBL. After a review of available methods, we present new results from an experiment by Mattila and Schnur (1989) utilizing the dark cloud technique in the area of L1642, a high-latitude dark nebula in the galactic anticentre direction.

In this review we first discuss the various ingredients of the non-evolving models for galaxy count observations. These ingredients include the K-corrections, the galaxy luminosity function and its dependence on galaxy colour, the local space density of galaxies and the cosmological deceleration parameter. Comparing the updated count model with the most recent galaxy count observations, the conclusion is still that at B>22m the B (and the R) counts require very significant amounts of evolution to fit the observed, steep n(m) relations. We review models of galaxy luminosity evolution and also discuss the current debate as to whether B≈23m galaxies have z ≈1 or z ≈3.

We also consider recent suggestions that the form of Tyson's counts at B=27m may constrain the cosmological deceleration parameter; contrary to the suggestion of Koo (1989), it is proposed here that if the turnover seen in Tyson's B counts is real then this argues for high values of qo and/or low galaxy formation redshifts. Finally, we consider the implications of these results for observations of the extragalactic background light (EBL) and conclude that in the ultraviolet (UV) and in the optical the contribution of faint galaxies to the EBL is likely to be small if the turnover in Tyson's counts is real. The galaxy count evidence gives less clear indications about the contribution of galaxies to the infrared EBL.

I examine some common features of several extragalactic backgrounds, beginning with the best studied of all backgrounds, the 3 K microwave radiation, then continuing on to the newly discovered submillimeter flux and the near infrared (IR) background. Both the total surface brightness and fluctuations (or anisotropies) in the brightness of these backgrounds are considered. Some implications for cosmology and for astrophysics are mentioned.

Four topics are discussed. First, measurements of the autocorrelation function of the optical extragalactic sky background check the possibility that there is an appreciable contribution from young galaxies or from stars well outside normal galaxies. Second, large-scale fluctuations in the mass distribution are probed by the anisotropies of the X-ray and 2.7 K backgrounds. Third, scattering by plasma in young galaxies could affect the primeval anisotropy of the 2.7 K background radiation. Fourth, production of the heavy elements is expected to yield background radiation with a characteristic and perhaps detectable angular distribution.

Due mainly to the minimal contaminating effects of zodiacal light and direct stellar emission, the far UV wavelength band from 912 to à 2000 å is ideally suited, in principle at least, for an accurate measurement of a diffuse background component due to sources outside our own galaxy. The cosmological significance of such radiation is of great current interest as it certainly includes the cumulative line-of-sight effect of galaxies and quasars and may include emission from both a lukewarm intergalactic medium and decaying massive particles such as neutrinos, photinos, etc. The radiation required to maintain the IGM at its known high ionization level should also, in any case, appear clearly in this band due to redshift and lookback time effects, thereby providing a crucial clue as to its presently obscure origins. Just how accurately this component can be measured in practice, however, clearly depends on how well we understand the probably dominant galactic component from which it must be disentangled in one way or another except, possibly, at the galactic poles. The residual emission there in this band is on the order of a few ×102 photons cm−2 s−1 sr−1 å−1, of which perhaps as many as 50 units are almost certainly due to galaxies since the small-scale spatial fluctuations corresponding to this flux almost exactly mimic those expected from the known spatial distribution of galaxies. The rest must come from a presently uncertain source, most likely a residual tenuous dust layer at the galactic poles. This latter possibility is at least consistent with recent IRAS results on the diffuse IR background at 100 μm and very sensitive HI, 21 cm measurements in these regions, but an extragalactic origin cannot be presently ruled out. Higher spatial and spectral resolution observations throughout the entire far UV range planned for the near future from orbiting platforms are expected to resolve this last but critically important issue.

Infrared extragalactic background light plays an important role in the study of the early history of the universe, especially as a probe to search for the primeval galaxies. In the near-infrared region, UV and visible light emitted from high redshift galaxies could be observable. Measurement of the sky fluctuation at 2.2 μm gives a very low upper limit. The rocket observation of the near-infrared diffuse emission reveals isotropic emission which is possibly ascribed to an extragalactic origin. The observed brightness and fluctuation are not consistent with the standard scenario of the primeval galaxies. In the far-infrared region, integrated light of dust emission of the distant galaxies forms another cosmic background radiation. IRAS and the Nagoya-Berkeley rocket experiment found a clear correlation between HI column density and far-infrared sky brightness; however, there remains an uncorrelated isotropic emission component. If we ascribe this emission to extragalactic origin, a fairly big evolution effect is required. In the submillimeter region, excess emission over the 2.74K blackbody spectrum was found, which requires the vast energy generation in the early universe.

The background of discrete extragalactic radio sources is described from the point of view of its simplest properties: surface density and surface distribution. New considerations, particularly with regard to the latter, open the possibility of detecting Universal structure on many angular scales. At the same time, these considerations imply the need for greater sophistication in the interpretation of source counts and of the microwave background (MWBG) fluctuations.

We review the data on the spectrum and isotropy of the microwave background radiation and the astrophysical processes that may produce spectral distortions and anisotropies. As yet no fully satisfactory explanation has been found for the submillimeter excess observed by Matsumoto et al. (1988). The most precise data at λ > 1 mm disagree with nonrelativistic comptonization models which match the excess. Distortions produced by a very hot intergalactic medium yielding the X-ray background do not fit the submillimeter data. Very special requirements must be met for the interpretation in terms of high-redshift dust emission to work.

Reported anisotropies on scales of several degrees and of tens of arcsec may be produced, at least in part, by discrete sources. Because the best experiments at cm wavelengths are close to the confusion limit, they provide interesting information on the large-scale distribution of radio sources.

The long-standing problem of the origin of the extragalactic X-ray background (XRB) is reviewed. Although the shape of the spectrum in the 3–100 keV interval is suggestive of an optically thin bremsstrahlung at ~ 40 keV, the interpretation in terms of a hot intergalactic gas (IGG) requires a rather extreme energy supply and a gas density conflicting with the baryon density upper limit derived from primordial nucleosynthesis calculations in the standard hot big-bang model. A summary discussion of the estimated contributions from the integrated X-ray emission of known classes of extragalactic discrete sources at a reference energy of 2 keV is given. Although these estimates are still uncertain, the subtraction of a “minimum” contribution drastically modifies the 40 keV thermal shape, which is the prima facie evidence of a hot IGG. AGNs are the main contributors. Low luminosity AGNs (Seyfert type 1 nuclei) at redshift z = 1 − 2 may in fact saturate the 2 keV XRB, but their observed hard X-ray spectra are on the average unlike (much too steep) that of the XRB. This has led a number of authors to postulate new classes of sources and some exotic models which are briefly summarized. However, if a recently proposed unified scheme of AGNs holds, then the bulk of the XRB intensity can be explained independently of the observed spectral differences and with a mild cosmological evolution. The origin of the extragalactic γ-ray background is briefly commented upon in the concluding remarks.

By mosaicking many CCD frames together, we have constructed a large-scale (~ 1/2°) R-band map of the cD cluster Abell 2029. The map was flat from one edge to the other to about 0.05% of the night sky, which corresponds to vR ~30 mag arcsec2. Using a novel technique involving the pixel distribution function, we measured diffuse light in the cluster out to 450″ (500 h−1 Kpc) along the minor axis of the cluster. In the elliptical region from minor radius 100″ to minor radius 300″, the diffuse light corresponds to roughly 8% of the total cluster light. Data in other optical bands and on other clusters are in the process of being reduced. The applicability of the above technique to measurements of the fluctuations in the extragalactic background light (EBL) is discussed.