2007ApJ...658..929R

We present age and metallicity determinations based on narrowband continuum colors for the galaxies in the rich clusters A1185 and Coma. Using a new technique to extract luminosity-weighted age and [Fe/H] values for non-star-forming galaxies, we find that both clusters have two separate populations based on these parameters. One population is old (τ>11 Gyr) with a distinct mass-metallicity relation. The second population is slightly younger (τ~9 Gyr) with lower metallicities and lower stellar masses. We find detectable correlations between age and galaxy mass in both populations such that older galaxies are more massive and have higher mean metallicities, confirming previous work with line indices for the same type of galaxies in other clusters (Kelson et al.; Thomas et al.). Given the previously discovered correlation between galaxy mass and α/Fe abundance (a measure of the duration of initial star formation; Denicolo et al.; Sanchez-Blazquez et al.), we interpret our age-metallicity correlations to imply that cluster galaxies are coeval with varying durations for their initial bursts. Our results imply shorter durations for higher mass galaxies, in contradiction to the predictions of classic galactic wind models. Since we also find a clear mass-metallicity relation for these galaxies, we conclude that star formation was more efficient for higher mass galaxies, a scenario described under the inverse wind models (Matteucci). With respect to cluster environmental effects, we find there is a significant correlation between galaxy mean age and distance from the cluster center, such that older galaxies inhabit the core.