Massive star formation in galaxies with excess ultraviolet emission

From an analysis of almost 2000 GALEX images of galaxies with morphological types ranging from E to Sab, we have found a significant subset (28%) that show UV emission outside R25. We have obtained Hα imaging of ten such galaxies, and find 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 disk galaxies, although the star formation rates of the elliptical galaxies in our sample are well below 1M 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.