The Color-Magnitude Relation in Coma: Clues to the Age and Metallicity of Cluster Populations

We have observed three fields of the Coma Cluster of galaxies with a narrowband (modified Strömgren) filter system. Observed galaxies include 31 in the vicinity of NGC 4889, 48 near NGC 4874, and 60 near NGC 4839, complete to M 5500 = -18 in all three subclusters. Spectrophotometric classification finds all three subclusters of Coma to be dominated by red, E-type (elliptical/S0) galaxies with a mean blue fraction, f B , of 0.10. The blue fraction increases to fainter luminosities, possible remnants of dwarf starburst population or the effects of dynamical friction removing bright, blue galaxies from the cluster population by mergers. We find the color-magnitude (CM) relation to be well defined and linear over the range of M 5500 = -13 to -22. The observational error is lower than the true scatter around the CM relation, indicating that galaxies achieve their final positions in the mass-metallicity plane by stochastic processes. After calibration to multimetallicity models, bright elliptical galaxies are found to have luminosity-weighted mean [Fe/H] values between -0.5 and +0.5, whereas low-luminosity elliptical galaxies have [Fe/H] values ranging from -2 to solar. The lack of CM relation in our continuum color suggests that a systematic age effect cancels the metallicity effects in this bandpass. This is confirmed with our age index Δ( bz - yz ), which finds a weak correlation between luminosity and mean stellar age in elliptical galaxies such that the stellar populations of bright elliptical galaxies are 2–3 Gyrs younger than low-luminosity elliptical galaxies. With respect to environmental effects, there is a slight decreasing metallicity gradient with respect to distance to each subcluster center, strongest around NGC 4874. Since NGC 4874 is the dynamic and X-ray center of the Coma Cluster, this implies that environmental effects on low-luminosity elliptical galaxies are strongest at the cluster core compared with outlying subgroups.