Radiative Transfer Analysis of Far-UV Background Observations Obtained with the Far-Ultraviolet Space Telescope (FAUST)

In 1992 the Far-Ultraviolet Space Telescope (FAUST) provided measurements of the ultraviolet (140-180nm) diffuse sky background at high, medium, and low Galactic latitudes. A significant fraction of the detected radiation was found to be of Galactic origin, resulting from scattering by dust in the diffuse interstellar medium. To simulate the radiative transfer in the Galaxy, we employed a Monte Carlo model which utilized a realistic, non-isotropic radiation field based on the measured fluxes (at 156nm) and positions of 58,000 TD-1 stars, and a cloud structure for the interstellar medium. The comparison of the model predictions with the observations led to a separation of the Galactic scattered radiation from an approximately constant background, attributed to airglow and extragalactic radiation, and to a well constrained determination of the dust scattering properties. The derived dust albedo a = 0.45 +/- 0.05 is substantially lower than albedos derived for dust in dense reflection nebulae and star-forming regions, while the phase function asymmetry g = 0.68 +/- 0.10 is indicative of a strongly forward directed phase function. We show the highly non-isotropic phase function to be responsible, in conjunction with the non-isotropic UV radiation field, for the wide range of observed correlations between the diffusely scattered Galactic radiation and the column densities of neutral atomic hydrogen. The low dust albedo is attributed to a size distribution of grains in the diffuse medium with average sizes smaller than those in dense reflection nebulae.Comment: 35 pages, 10 figures included, to be published in the Ap

FAUST observations of UV sources toward the Virgo cluster

We analyze three UV images covering a 100 square degree field toward the Virgo cluster,obtained by the FAUST space experiment. We detect 191 sources to a signal-to-noise ratio of 4.4 and identify 94% of them. Most sources have optical counterparts in existing catalogs and about half are identified as galaxies. Some sources with no listed counterpart were observed at the Wise Observatory. We present the results of low resolution visible spectrophotometry and discuss the foreground 101 stellar sources and the 76 detected galaxies, both in the cluster and in the fore- or background. We derive conclusions on star-formation properties of galaxies and on the total UV flux from discrete and diffuse sources in the cluster. We test for the presence of intra-cluster dust, determine the clustering properties of UV emitting galaxies, and derive the UV luminosity function of Virgo galaxies.Comment: 60 pages, LaTeX (including 5 Tables) + 8 PostScript figures. To appear in "The Astrophysical Journal