The ultraviolet spectrum of the center of M 31.

Abstract

A total of twelve long-exposure low-resolution IUE images have been used to study the spectrum and surface brightness distribution of the ultraviolet radiation within the central 50 pc of M31. The presence of weak absorption features near 1300 A demonstrates that the radiation of the well-known spectral upturn below 2000 A does not originate in young metal-rich stars. The most likely sources are considered to be hot, highly evolved stars. The energy distribution is bluer than that measured by experiments with larger entrance apertures, and below 2000 A the surface brightness gradient near the nucleus increases. These findings indicate that the blue stars are more centrally concentrated within the bulge of M31 than the more metal-rich red giants. The energy distribution measured by IUE appears to be much bluer at short wavelengths than that of the metal-rich globular cluster 47 Tucanae, yet it also differs from those of clusters having more populous blue horizontal branches.