GALEX Observations of Diffuse UV Radiation at High Spatial Resolution from the Sandage Nebulosity

TL;DR

This study uses GALEX UV imaging to analyze diffuse UV radiation in a nebula, revealing contributions from dust scattering, H2 fluorescence, and extragalactic sources, with high spatial resolution data showing intrinsic scatter.

Contribution

First high-resolution GALEX observations of diffuse UV background separating dust scattering, H2 fluorescence, and extragalactic contributions.

Findings

Diffuse UV background between 500-800 phu in FUV and NUV.

Up to 250 phu from H2 fluorescence in FUV.

Highly forward scattering grains with optical constants a=0.3-0.5, g=0.7.

Abstract

Using the GALEX ultraviolet imagers we have observed a region of nebulosity first identified as starlight scattered by interstellar dust by Sandage (1976). Apart from airglow and zodiacal emission, we have found a diffuse UV background of between 500 and 800 \phunit in both the \galex FUV (1350 -- 1750 \AA) and NUV (1750 -- 2850 \AA). Of this emission, up to 250 \phunit is due to \htwo fluorescent emission in the FUV band; the remainder is consistent with scattering from interstellar dust. We have estimated the optical constants to be $a = 0.3; g = 0.7$ in the FUV and $a = 0.5; g = 0.7$ in the NUV, implying highly forward scattering grains, plus an extragalactic contribution of as much as 150 \phunit. These are the highest spatial resolution observations of the diffuse UV background to date and show an intrinsic scatter beyond that expected from instrumental noise alone. Further modeling is required to understand the nature of this scatter and its implications for the ISM.