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 Stromgren) filter system. Observed galaxies include 31 in thevicinity of NGC 4889, 48 near NGC 4874, and 60 near NGC 4839 complete toM_5500=-18 in all three subclusters. Spectrophotometric classification findsall three subclusters of Coma to be dominated by red, E type (ellipticals/S0's)galaxies with a mean blue fraction, f_B, of 0.10. The blue fraction increasesto fainter luminosities, possible remnants of dwarf starburst population or theeffects of dynamical friction removing bright, blue galaxies from the clusterpopulation by mergers. We find the color-magnitude (CM) relation to be welldefined and linear over the range of M_5500=-13 to -22. After calibration tomulti-metallicity models, bright ellipticals are found to have luminosityweighted mean [Fe/H] values between -0.5 and +0.5, whereas low luminosityellipticals have [Fe/H] values ranging from -2 to solar. The lack of CMrelation in our continuum color suggests that a systematic age effect cancelsthe metallicity effects in this bandpass. This is confirmed with our age indexwhich finds a weak correlation between luminosity and mean stellar age inellipticals such that the stellar populations of bright ellipticals are 2 to 3Gyrs younger than low luminosity ellipticals.Comment: 26 pages AAS LaTeX, 6 figures, accepted for publication in A