THE DIFFUSE GALACTIC FAR-ULTRAVIOLET SKY

Abstract

We present an all sky map of the diffuse Galactic far ultraviolet (1344-1786 Angstroms) background using GALEX data, covering 65% of the sky with 11.79 arcmin square pixels. We investigate the dependence of the background on Galactic coordinates, finding that a standard cosecant model of intensity is not a valid fit. Furthermore, we compare our map to Galactic all sky maps of 100 micron emission, N_HI column, and H-alpha intensity. We measure a consistent low level FUV intensity at zero-points for other Galactic quantities, indicating a 300 CU non-scattered isotropic component to the diffuse FUV. There is also a linear relationship between FUV and 100 micron emission below 100 micron values of 8 MJy/sr. We find a similar linear relationship between FUV and NHI below 10^21 square cm. The relationship between FUV and H-alpha intensity has no such constant cutoff. For all Galactic quantities, the slope of the linear portion of the relationship decreases with Galactic latitude. A modified cosecant model, taking into account dust scattering asymmetry and albedo, is able to accurately fit the diffuse FUV at latitudes above 20 degrees. The best fit model indicates an albedo, a, of 0.62 +- 0.04 and a scattering asymmetry function, g, of 0.78 +- 0.05. Deviations from the model fit may indicate regions of excess FUV emission from fluorescence or shock fronts, while low latitude regions with depressed FUV emission are likely the result of self-shielding dusty clouds.