Age and Metallicity of Elliptical Galaxies
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Session 42 -- Ellipticals
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[42.06] Age and Metallicity of Elliptical Galaxies

J. Jes\'us Gonz\'alez (IA-UNAM), Sandra M. Faber (UCO/Lick Obs.), Guy Worthey (Univ. of Michigan)

We have measured a set of high-quality spectral indices for a sample of 40 normal elliptical galaxies. To study the stellar population in these systems, the line-strengths were measured with the Lick/IDS system and compared to model predictions for co-eval generations of stars as a function of age and metallicity. Strengths of features from species like Mg and Fe are not enough to separate age and metallicity of composite stellar populations. The age-metallicity degeneracy has been broken in this study by including the $H_\beta$ strength (an index more sensitive to age than metallicity). The $H_\beta$ strength varies considerably among the galaxies in contrast to the more metallicity-sensitive indices. The well defined light-strength patterns of the data reflect remarkable age-metallicity trends when compared to the models: although the mean stellar metallicity does not vary significantly among strong and weak-lined galaxies ($\Delta \log \overline{Z} \simlt .18$ dex), these two types of galaxies are separated by a considerable age spread. The strong $H_\beta$ absorption of weak-lined galaxies implies mean stellar ages as small as $\sim 3-6$ Gyr for these systems, in contrast to ages of $\sim 12-15$ Gyr in strong-lined galaxies. The Mg and Fe line-strength indices indicate a high [Mg/Fe] abundance ratio in strong-lined galaxies relative to the calibrating stars.

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