AAS 200th meeting, Albuquerque, NM, June 2002
Session 40. Evolution of Galaxies, Galaxy Surveys, IGM
Display, Tuesday, June 4, 2002, 10:00am-6:30pm, SW Exhibit Hall

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[40.16] Ages and Metallicities of Extragalactic Globular Clusters and Galaxies from Near-UV Spectra

R. C. Peterson (Astrophysical Advances and UCO/Lick), B. W. Carney (U North Carolina), B. Dorman (Emergent IT), W. Landsman (SSAI), E. M. Green, J. Liebert (U Arizona), R. W. O'Connell, R. T. Rood (U Virginia)

We are embarking on a three-year Hubble Treasury program, GO-9455, whose goal is to place the determination of ages and metallicities old stellar systems on a quantitative basis. Our program will provide the tools to analyze their composite integrated-light spectra in the near ultraviolet (UV). We will calculate an extensive grid of stellar spectra from 2280A to 3120A from first principles, then construct grids of composite spectra by coadding the stellar spectra using weights determined from stellar isochrones, and generate spectral indices from the stellar and composite spectra. We will improve the near-UV line list during the first two years, then calculate the theoretical spectra and apply them to Andromeda globular clusters in the third year. Our Space Telescope Imaging Spectrograph near-UV spectra of stars and globulars will be made publicly available as soon as they are taken, and our stellar and composite grids and indices will be publicly distributed as they are completed.

Here we illustrate the application of theoretical stellar spectra to the understanding of the population components of the globular cluster G1 = Mayall II in the nearby Andromeda galaxy (M31). We coadd spectral fluxes of individual stellar models, choosing weights from the observed color-magnitude diagram of the Galactic globular 47 Tucanae, and compare these to the Hubble Faint Object Spectrograph spectra in the near-ultraviolet and visible obtained by Ponder et al. (1998, AJ, 116, 2297). We find that the rise in the observed spectra below 2400A cannot be modeled using spectra from main-sequence stars of temperatures cooler than 7500K. Instead, we obtain an excellent fit by including spectra of cool blue horizontal branch (BHB) stars, with temperature 8000K and approximate spectral type A7III. These BHB stars contribute 50% of the light at 2600A.

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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.