HST Far–UV Imaging of M 31, M 32 and NGC 205

Abstract

Direct measures of luminosity functions, UV fluxes and colors of the hot stars that produce the UV turn–up in the SED of ellipticals and spiral bulges is the scientific goal of the HST observations presented here (see Bertola et al. 1995 for details). We concentrated our analysis on the hot stellar content of the M31 bulge. HST/FOC f/48 images were analyzed as observed (before the repair mission) through the F150W broad-band UV filter. We find that both individual stars and unresolved objects contribute to about 50% of the UV (1200-2450 Å) flux. Making use of the isochrones calculated by Bertelli et al. (1994) and Chiosi et al. (1994) together with the models of population synthesis presented by Bressan et al. (1994), we constructed theoretical CMDs to be compared with the CMD obtained by combining our F150W observations with the F175W observations of King et al. (1992). We conclude that the stars we resolved in M31 are classical P-AGB stars belonging to an old standard metallicity population. However, the P-EAGB and AGB-manqué stages as well as H-HB stars could be the source of the diffuse UV emission.