Low-Luminosity Early-Type Galaxies in the Coma Cluster: Variations in Spectral Properties

We present spectra of nine low-luminosity early-type galaxies (MB > −17.5) in the Coma cluster. The spectra, which were obtained with the Multiple Mirror Telescope and cover the spectral region 3300-5350 Å, exhibit a large variation in Balmer line strengths. In fact the line strength variation among the Coma low-luminosity galaxies is as large as that found among a sample of low-luminosity ellipticals in the Virgo cluster and lower density environments. When compared to detailed population synthesis models, the variation in Balmer line strengths among the Coma galaxies indicates a range in luminosity-weighted mean age of from ∼1 Gyr to ∼12 Gyr. The two youngest galaxies are shown to be in a post-starburst state, i.e., they are not simply former spirals whose star formation was recently terminated. Moreover, the ∼1 Gyr ages of these two youngest low-luminosity galaxies are similar to those of the brighter post-starburst galaxies in Coma.