Massive star formation in galaxies with excess UV emission

Abstract

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 $R_{25}$. We have obtained H$\alpha$ imaging of ten such galaxies, and find that their star formation rates are similar in the UV and in H$\alpha$, with values ranging from a few tenths to a few $M_{\odot} $ 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 $1\,M_{\odot} $ 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.