Hot stars in the bulge of M31 - Upper limit to the star formation rate

Abstract

IUE satellite UV observations of the bulge in M 31 indicate upon comparison with previous photometric observations that the far-UV surface brightness distribution is probably similar to that in the B light, despite the discovery of a gradient in (UV-B) color at the very center of this galaxy. While no firm explanation is found for the observed UV color gradient, hot evolved stars are suggested as the major source of the UV flux. Comparison with evolutionary models gives a star formation rate in the bulge of M 31 of 0.000074 solar masses/year, which is inadequate for the consumption of gas shed by dying stars, while examination of NGC 205 leads to the opposite conclusion.