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Session 49 - Elliptical Galaxies & Bulges.
Display session, Thursday, January 08
Exhibit Hall,

[49.03] The Mid-Ultraviolet Spectral Range of Old Stellar Populations

B. Dorman (U. Virginia and NASA/GSFC), R. W. O'Connell, R. T. Rood (U. Virginia)

We present a report on our studies of the properties of the mid-ultraviolet (2200 < \lambda < 3200 Åcontinuum and spectral features of elliptical galaxies. This spectral range is particularly important as it is contributed almost entirely by the main-sequence turnoff, which both defines and is sensitive to the age and metallicity of the bulk stellar population present. In addition, it is readily observable from the ground at z > 1, and it is important to develop tools for its proper interpretation to study ellipticals at significant lookback times.

We have produced model spectral energy distributions from stellar isochrones computed using a new interpolation code, and the horizontal branch tracks of Dorman, Rood and O'Connell (1993). We address several issues:

\beginenumerate \item the sensitivity of the mid-UV continuum to age and metallicity and the prospects for resolving the ``age-metallicity degeneracy" problem using UV observations; \item the observational accuracy required to derive a conclusion concerning gross galaxy properties from the data; \item a technique for removing the contamination due to the hot component that is responsible for the Ultraviolet Upturn phenomenon at shorter UV wavelengths from the observation, and studying the underlying light from the cooler stellar population. \endenumerate

We present fits to galaxy spectra (M31, M32, M60, and M89) using our technique. We find that the continua of the galaxies are consistent with solar metallicity populations: in the case of M32, the best fit to the UV/Optical colors is about 7 Gyr, while the other galaxies have implied ages of 12-15 Gyr. Various features of the mid-UV spectra, however, suggest the presence of components warmer than those implied by UV/Optical colors.

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