A photometric and spectroscopic study of dwarf and giant galaxies in the Coma Cluster. III. Spectral ages and metallicities

We present a detailed analysis of the spectroscopic catalog of galaxies in the Coma Cluster from Mobasher et al. (Paper II of the series). This catalog comprises similar to 300 spectra of cluster members with absolute magnitudes in the range M-B = -20.5 to -14 in two areas of similar to1 x 1.5 Mpc toward the center and the southwest region of the cluster. In order to study the star formation and metallicity properties of the Coma galaxies as a function of their luminosity and environment, spectral indices of the Lick/IDS system and equivalent widths of the emission lines were measured in the range lambda = 3600-6600 Angstrom. In this paper, the analysis is restricted to the 257 galaxies with no emission lines in their spectra. The strength of the age-sensitive indices (such as H beta, H gamma (F), and H delta (F)) is found to correlate with galaxy magnitude over the whole magnitude range explored in this study. Similarly, the metallicity-sensitive indices (such as Mg-2, [Fe], and C(2)4668) anticorrelate with magnitude. By comparing the observed indices with model grids based on the Padova isochrones, we derive luminosity-weighted ages and metallicities. We present the distributions of ages and metallicities for galaxies in various magnitude bins. The mean metallicity decreases with galaxy magnitude and, at a given luminosity, appears to be generally lower for galaxies in the southwest region of Coma as compared to the center of the cluster. A broad range of ages, from younger than 3 Gyr to older than 9 Gyr, is found in galaxies of any magnitude. However, systematic trends of age with luminosity are present among galaxies in the central field, including a slight decrease of the mean age for fainter galaxies. Furthermore, in the central Mpc of Coma, a large fraction of galaxies at any luminosity (50%-60% of the giants, more than 30% of the dwarfs) show no evidence in their central regions of star formation occurred at redshift z < 2, while the proportion of galaxies with significant star formation occurring at intermediate (0.35 < z < 2) and low (z < 0.35) redshifts is found to depend on galaxy luminosity. An additional surprising result is that the faint galaxies with young luminosity-weighted ages appear to have a bimodal metallicity distribution that, if confirmed, would point to a composite formation scenario involving different physical processes. Coadding the spectra of these metal-rich and metal-poor galaxies separately supports the reality of the metallicity bimodality, although higher signal-to-noise ratio spectra of the individual galaxies will be needed to draw definite conclusions. An anticorrelation between age and metallicity is found to be present in galaxies of any given luminosity bin, and it is especially evident among the brightest subset with the highest signal-to-noise ratio spectra. Finally, we present an interpretation of the index-magnitude relations observed. We show that the slopes of the indices/magnitude relations are the consequence of both age and metallicity trends with luminosity : each such trend on its own would be sufficient to produce relations similar to those observed.