Observations of Diffuse Ultraviolet Emission from Draco

TL;DR

This study analyzes small-scale variations in diffuse UV emission in Draco, revealing dust-scattered starlight and additional molecular hydrogen and line emissions, with quantification of extragalactic light contributions in UV bands.

Contribution

First detailed analysis of small-scale UV variations in Draco, identifying specific emission sources and quantifying extragalactic contributions in UV background.

Findings

UV background correlates with IR 100 micron flux.

Evidence of molecular hydrogen and line emissions in FUV.

Extragalactic light contributes ~49% in NUV and ~30% in FUV.

Abstract

We have studied small scale (2 arcmin) spatial variation of the diffuse UV radiation using a set of 11 GALEX deep observations in the constellation of Draco. We find a good correlation between the observed UV background and the IR 100 micron flux, indicating that the dominant contributor of the diffuse background in the field is the scattered starlight from the interstellar dust grains. We also find strong evidence of additional emission in the FUV band which is absent in the NUV band. This is most likely due to Lyman band emission from molecular hydrogen in a ridge of dust running through the field and to line emissions from species such as C IV (1550 A) and Si II (1533 A) in the rest of the field. A strong correlation exists between the FUV/NUV ratio and the FUV intensity in the excess emission regions in the FUV band irrespective of the optical depth of the region. The optical depth increases more rapidly in the UV than the IR and we find that the UV/IR ratio drops off exponentially with increasing IR due to saturation effects in the UV. Using the positional details of Spitzer extragalactic objects, we find that the contribution of extragalactic light in the diffuse NUV background is 49 +/- 13 photon units and is 30 +/- 10 photon units in the FUV band.