Massive star formation in galaxies with excess ultraviolet emission

Abstract

From an analysis of almost 2000 Galaxy Evolution Explorer images of galaxies with morphological types ranging from E to Sab, we have found a significant subset (28 per cent) that show ultraviolet (UV) emission outside R25. We have obtained Hα imaging of 10 such galaxies, and found that their star formation rates are similar in the UV and in Hα, with values ranging from a few tenths to a few M⊙ yr−1. Probably because our sample selection is biased towards star-forming galaxies, these rates are comparable to those found in disc galaxies, although the star formation rates of the elliptical galaxies in our sample are well below 1 M⊙ yr−1. We confirm that the extended UV emission in our sample is caused by massive star formation in outer spiral arms and/or outer (pseudo) rings, rather than by alternative mechanisms such as the UV upturn.