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abstract
The infamous degeneracy of the effects of age and metal abundance on the integrated light of old stellar populations has been broken. The H$\gamma _{HR}$ spectral index is shown to be almost exclusively sensitive to age and when combined with any of several metallicity indicators can cleanly separate the effects of age and metal abundance in integrated light. The spectral indices are part of a new system, developed in collaboration with Guy Worthey, combining the Lick and Rose index systems via the evolutionary synthesis models of Worthey. The models predict an index value for a population of given age and metal abundance providing the ability to compare measured indices from real galaxies with the model populations. We have noticed a large dispersion in the spectral properties of low-luminosity elliptical galaxies and have defined the H$\gamma _{HR}$ index to capitalize on the considerable line definition available in systems with low internal velocity dispersions ($\sigma \le 120\ km/sec$). We present a comparison of the stellar populations in the galaxies M32 and NGC 3605 as an example of the spread in properties of low-luminosity ellipticals.
