Mapping the Diffuse Ultraviolet Sky with GALEX

Abstract: We present a map of the diffuse ultraviolet cosmic background in two wavelength bands (FUV: 1530 {\AA}; NUV: 2310 {\AA}) over almost 75% of the sky using archival data from the 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 {\AA}-1 and 433 photons cm-2 s-1 sr-1 {\AA}-1 in the FUV and NUV bands, respectively. There is a strong correlation with the 100 {\mu}m IRAS flux with an average UV/IR ratio of 300 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the FUV band and 220 photons cm-2 s-1 sr-1 {\AA}-1 (MJy sr-1)-1 in the NUV 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.