Mapping the Diffuse Ultraviolet Sky with the Galaxy Evolution Explorer

Abstract

We present a map of the diffuse ultraviolet cosmic background in two wavelength bands (FUV: 1530 Å and NUV: 2310 Å) over almost 75% of the sky using archival data from the Galaxy Evolution Explorer (GALEX) mission. Most of the diffuse flux is due to dust-scattered starlight and follows a cosecant law with slopes of 545 photons cm–2 s–1 sr–1 Å–1 and 433 photons cm–2 s–1 sr–1 Å–1 in the FUV and NUV bands, respectively. There is a strong correlation with the 100 μm Infrared Astronomy Satellite (IRAS) flux with an average UV/IR ratio of 300 photons cm–2 s–1 sr–1 Å–1 (MJy sr–1)–1 in the FUV band and that of 220 photons cm–2 s–1 sr–1 Å–1 (MJy sr–1)–1 in the NUV band but with significant variations over the sky. In addition to the large-scale distribution of the diffuse light, we note a number of individual features including bright spots around the hot stars Spica and Achernar.