Radiative Transfer Analysis of Far‐Ultraviolet Background Observations Obtained with the Far Ultraviolet Space Telescope

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