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K. D. Concannon (UNC-CH), N. Caldwell (SAO), J. A. Rose (UNC-CH)
We present new evidence that low-mass early-type galaxies exhibit more varied star formation histories than their brighter counterparts. Using spectral line diagnostics which are sensitive to the age and metallicity variations within the galaxy sample, we find that for a sample of 100 field and cluster E/S0 galaxies, the lower mass systems exhibit a larger range in Balmer line strengths. The observed spread in line strength indicates that the low mass galaxies have experienced more recent star formation episodes and a more varied star formation history than the higher mass galaxies. Furthermore, the measured strength of the Balmer lines correspond to a wide range in age with a significant fraction of the low mass galaxies having experienced star formation within the past few Gyr. Determining the population ages observed in these low mass galaxies can provide observational constraints to theories such as the hierarchical model which govern the formation and subsequent evolution of large-scale structures.
Additionally, we report on the strengths of three of the spectral indicators used in our analysis --- Mg2, H\beta, and Hn/Fe (a composite measure of three higher order Balmer lines) --- and show that there is significantly more intrinsic scatter, at a given velocity dispersion, in the two Balmer line indices than in the Mg2 index. Our principal conclusion from this analysis is that the higher order Balmer lines provide a sensitive and incredibly promising galaxy age indicator.
The author(s) of this abstract have provided an email address for comments about the abstract: dendy@physics.unc.edu