We present an all-sky star count model at 12 μm based upon the Infrared Astronomical Satellite (IRAS) observations that characterize both the 12 μm luminosity function and the geometrical parameters of the galaxy. The model includes five galactic components: the bulge, the spheroid, the exponential disk, the spiral arms, and the molecular ring. The distribution of the brighter IRAS sources along the galactic plane required that the model include sources within the spiral arms and the molecular ring to produce an acceptable fit. We do not support the conclusion of Habing (1988) that the galactic disk ends just outside the solar circle, and do not require a thick disk to match the observations. We suggest that Habing's sample includes IRAS sources in the spiral arms but his model for the galactic disk does not include this critical component.

From the IRAS catalogue we selected 10,001 young infrared (IR) sources which lie within the galactic plane. Their IR flux integrated over ±5° in latitude shows maxima at galactic longitudes l = 80°, 60°, 50°, 35°, −27°, −54°, and −74°, which directions are interpreted as tangents to the spiral arms. The resulting spiral arm pattern is nearly identical with the arms derived from observations of O and B stars.

The diffuse galactic light, resulting from the coherent scattering of galactic starlight by dust grains contained in the general interstellar medium, has been the subject of active investigation for nearly 60 years. The separation of the diffuse galactic light from the other sources contributing to the light from the night sky has proven difficult, and different attempts at measuring the intensity and galactic distribution of the diffuse galactic light, both in the visible and the UV, are reviewed here. The interpretation of such measurements in terms of average scattering properties of interstellar grains is subject to additional uncertainties, stemming from the high degree of idealization imposed on the galaxy models used to study the radiative transfer problem. In the visible, the observations are more nearly definitive and the model problems less severe; reasonably consistent scattering properties have therefore been derived for this spectral region. In the UV, the situation is considerably less satisfactory, mainly due to a lack of sufficiently extensive, reliable measurements of the diffuse galactic light intensity at λ < 200 nm. A dedicated space mission may be the required solution. The radiative transfer in the UV presents serious difficulties due to the increasing opacity of the interstellar medium with shorter wavelengths and the resulting growing importance of the local galactic structure.

Results obtained with the Infrared Astronomy Satellite (IRAS) on the IR emission at high galactic latitude are reviewed. We present evidence for the detection of galactic emission at 12, 25, 60, and 100 μm. We describe the morphology of this emission and summarize work on the correlation of IR cirrus with H I, CO, and optical emission. We discuss the contribution of the neutral atomic, ionized, and molecular gas to the total IR emission and the contribution of different components of interstellar dust to the emission seen at different wavelengths.

Diffuse infrared background radiation measurements have traditionally been carried out across broad wavelength bands, primarily because there seldom appeared to be sufficient energy available in individual spectral lines to make such observations possible. In the past few years, far-infrared searches for spectral line background radiation have become possible, and considerably greater use of these capabilities will become available in the next few years. The promise of this kind of observation is reviewed.

Diffuse galactic Hα emission appears to cover the entire sky with an intensity that ranges from 3–12 R near the galactic equator to 0.25–0.8 R near the galactic poles. Observations of this H-recombination line and the forbidden lines, [S II] 6716 Å, [N II] 6583 Å, and [O III] 5007 Å, indicate that the emission originates from a low-density, 2–3 kpc thick layer of warm (~104 K), ionized interstellar gas that has an emission-line spectrum significantly different from that of the traditional, more localized H II regions. Along a line perpendicular to the galactic disk, the mean emission measure of this layer is 4.5 cm−6 pc, and the column density of the H+ is 2 × 1020 cm−2. The origin of this diffuse ionization is not yet clear; however, its existence requires the equivalent of about 14% of the total ionizing photon flux from O stars or nearly all of the power injected into the ISM by supernova. This optically emitting gas also may be a nonnegligible source of diffuse emission in the far ultraviolet (FUV) and infrared (IR).

Measurements of the far ultraviolet background are reviewed. A major turning point occurred in the study of this field in the early 1980s, when evidence was first presented that this flux was primarily galactic in origin rather than extragalactic, as had been generally believed. A number of experiments have confirmed this result, and it has been established that the flux is the result of scattering of starlight by dust. However, the detailed scattering properties of dust in the far ultraviolet are uncertain; a wide range of albedos and scattering phase functions have been reported. Very recent evidence indicates that ultraviolet scattering grains are different from grains that scatter in the visible in that they have a low albedo and scatter isotropically. There is evidence that this dust is present at some level in all view directions in the galaxy. Spectral emission features have been detected recently in the diffuse background. Lines of C IV and O III] have been observed and lines of O IV/Si IV and N III have probably been observed. It has been established that the 105 K gas producing these lines is 2–3 kpc above the galactic plane. Overall mass flux rates of 5 to 25 M⊙ yr−1 for this gas are indicated, which provides strong support for the galactic fountain model for this material. Emission from molecular hydrogen has been detected in directions of high and low neutral hydrogen column density. This emission emanates from low density molecular clouds and indicates clumping of the emitting material in the clouds. Our knowledge of the sources of the far ultraviolet background has increased dramatically in the past 10 years. The results obtained have yielded surprising new insights on a variety of astrophysical topics.

The correlations between the UV galactic background (after subtraction of direct starlight), the neutral hydrogen HI, the extinction and the 100 μm emission are discussed for high galactic latitudes mainly based on a new analysis of the D2B satellite data. While clear correlations exist between UV and the other quantities, indicating that most of the UV galactic background is starlight scattered by interstellar dust, these correlations are less tight than the correlation between HI and the 100 μm emission. Localized UV excesses are shown to exist and are due in some cases to 2-photon emission by ionized gas. The quantitative interpretation of the UV galactic background in terms of dust properties is discussed, and it is shown that both the galactic model and the clumpiness of interstellar radiation may affect the results considerably.

The measurement of Hα emission on two high galactic latitude clouds known to emit 21 cm lines with local standard of rest (LSR) velocities of −50 and −85 km s−1 is reported. The Hα lines have been found nearly at the velocities of the 21 CM features and have emission rates of 0.2 Rayleigh. The diffuse Lyman continuum intensity required to produce HI ionization at the measured rate is in agreement with the direct measurements made with the ultraviolet (UV) spectrometer of the Voyager 2 spacecraft.

The purpose of the present investigation was to find out how far the existing photographic sky surveys can be used to derive physical parameters of dark clouds, without the need of new observations. In a recent paper a method of distance determination and extinction mapping of dark clouds by automated star counts on microphotometric raster scans of the ESO/SRC Southern Sky Survey (Stüwe, 1989, hereafter Paper I) is presented. The present paper does a second step, which is based on the fact that dark clouds are marked by an intrinsic surface brightness (Barnard, 1927).

High resolution (0.°25 × 0.°25) surface brightness distribution in V of the southern Milky Way over an area of 200° ≤ l ≤ 60° and of −30° ≤ b ≤ +30° was obtained by photographic plates, taken at La Silla, Chile, with the super-wide-angle camera with spherical mirror of the Astronomisches Institut der Ruhr-Universität Bochum (Schmidt-Kaler et al. 1983). Schmidt-Kaler et al. (1983) and Seidensticker, Schmidt-Kaler, and Schlosser (1982) carried out an analysis of these plates. However, these studies used only a fraction of the whole plate; interesting parts of the sky were chosen near the plate centers, thus minimizing various errors. The plates are now all scanned over the whole field of view with a PDS with a diaphragm of 50 × 50 μm = 0.°12 × 0.° 12 on the sky. The image size is 1201 × 1201 pixels per plate. The mean deviation during the scan time was less than 0.1%. Through the identification of about 50 stars and by using their l, b and x, y coordinates, the equations of the plates were solved with eight geometric parameters. The standard deviation of all parameters was less than 0.3 pixels on all plates.

During the last two decades, many attempts were made to determine the global parameters of the Galaxy and to compare the Galaxy to other galaxies (Schmidt-Kaler and Schlosser 1973; de Vaucouleurs and Pence 1978; Gilmore 1984; van der Kruit 1986). While most of these investigations are based on star counts, a detailed overall study by surface photometry, because of the lack of homogeneous high-resolution data, is rare. The last attempt by van der Kruit (1986), based on Pioneer 10 data, suffered from low resolution. The great number of individual structures at low and even intermediate latitudes could not be recognized. Our work (B-band, Hoffmann et al. 1989, this volume; V-band, Schlosser, Schmidt-Kaler, and Schneider 1989; U-Band and R-band photometry, in preparation) provides this homogeneous high-resolution data.

An extensive survey of [CII] line emission has been made with a balloon-borne infrared telescope. It has been found that the emission is diffuse and ubiquitously distributed in general interstellar space.

Variations in the optical surface brightness of the night sky on angular scales of arcseconds to degrees are contributed to by phenomena ranging from geophysics to cosmology. Known time-independent (over years) sources include (1) fluctuations in the distant galaxy distribution and (2) scattered disk light off dust in the plane of our galaxy and at high latitudes (IRAS cirrus; Low et al. 1984). Sandage (1976) found optical “cirrus” on photographic plates, which were later correlated with bright 100 μm cirrus (de Vries and Le Poole 1985). We describe here a systematic investigation of fainter and more widespread high-latitude IRAS cirrus using deep multicolor charge-coupled device (CCD) imaging photometry (Guhathakurta and Tyson 1989).

AROME∗ is a balloon-borne experiment which was built to carry out measurements of IR emission features in the diffuse galactic flux. The field of view is 0.5° and surface brightness gradients are detected through azimuthal scanning at a constant elevation angle. The detection of a feature is done by comparison of the fluxes measured in narrow and wide photometric bands centered on the feature's wavelength. Two flights have been performed (August 1987, October 1988), which detected a 3.3 μm feature in the direction of the galactic plane −6° < b < 6°, 60° > l > −50°. Since this feature is characteristic of aromatic C-H bonds, we assigned it to the emission of transiently heated polycyclic aromatic hydrocarbon molecules (PAHs). With this assumption, AROME measurements show that PAHs are an ubiquitous component of the interstellar matter which contain about 10% of the available cosmic carbon.

We have tested the performance of principal components analysis and a single-pass clustering algorithm to identify different components of the cosmic dust. Applying these techniques on a training set of 2500 points extracted from the PL51 IRAS maps we recognized two main components with temperatures of 180 K and 28 K.

The high-galactic-latitude cloud L1642 (l = 210.8°, b = −36.7°) is a suitable candidate to relate IR measurements with atomic and molecular data because it has a reasonable size with respect to the rather poor (IRAS) IR resolution, a moderate optical extinction and an isolated location in the direction towards the galactic anticenter. The exceptionally high galactic latitude of −36.7° implies that L1642 is some 60 pc below the galactic plane (if r ≈ 100 pc is adopted for its distance). L1642 is thus sufficiently far off the galactic plane to minimize the confusion by background gas and dust clouds.

Prerequisite to the determination of the Galactic and Extragalactic contributions to the infrared background is an accurate removal of the zodiacal emission. The all-sky observations by IRAS in 1983 were made with a variety of observing conditions and with higher sensitivity and angular resolution than previous infrared and optical observations of the zodiacal background. These facts alone mandate a better understanding of the optical properties and spatial distribution of interplanetary dust.

Deep spectra of the far-ultraviolet sky background have been obtained by the Voyager 2 Ultraviolet Spectrometer (UVS) in the 500 to 1200 Å wavelength range. New diffuse galactic background results—particularly in the 900 to 1100 Å region—are presented, which extend the North Galactic Pole results of Holberg (1986) over a range of lower galactic latitudes. Twenty-four lines of sight are analyzed for the presence of residual diffuse continua. Typical upper limits of several hundred photons cm−2 s−1 Å−1 sr−1 are obtained for most lines of sight; however, a faint residual continuum is detected at several low-latitude locations. In Ophiuchus, a diffuse stellarlike continuum is attributed to scattering from interstellar dust.

We report on results from the Berkeley Ultraviolet Experiment (UVX), which performed 15 ± 2 Å resolution spectroscopy of the diffuse far ultraviolet background in eight directions. We have used the spectrum obtained in the direction of low H I column density to derive constraints on any extragalactic background. We find evidence that a hitherto unidentified dust component is present that accounts for most of the background in directions of low neutral hydrogen column density.