The Galactic Far Ultraviolet Background

Abstract

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.