The UV spectrum of the Galactic bulge

Abstract

The UV excess shown by elliptical galaxies in their spectra is believed to be caused by evolved low-mass stars, in particular sdB stars. The stellar system most similar to the ellipticals for age and metallicity, in which it is possible to resolve these stars, is the bulge of our Galaxy. sdB star candidates were observed in the color magnitude diagram of a bulge region by Zoccali et al. (2003). The follow-up spectroscopic analysis of these stars confirmed that most of these stars are bulge sdBs, while some candidates turned out to be disk sdBs or cool stars. Both spectroscopic and photometric data and a spectral library are used to construct the integrated spectrum of the observed bulge region from the UV to the optical: the stars in the color magnitude diagram are associated to the library spectra, on the basis of their evolutionary status and temperature. The total integrated spectrum is obtained as the sum of the spectra associated to the color magnitude diagram. The comparison of the obtained integrated spectrum with old single stellar population synthetic spectra calculated by Bruzual & Charlot (2003) agrees with age and metallicity of the bulge found by previous work. The bulge integrated spectrum shows only a very weak UV excess, but a too strict selection of the sample of the sdB star candidates in the color magnitude diagram and the exclusion of post-Asymptotic Giant Branch stars could have influenced the result.