A Photometric and Spectroscopic Study of Dwarf and Giant Galaxies in the Coma Cluster. III. Spectral Ages and Metallicities

Abstract

We present a detailed analysis of the spectroscopic catalog of galaxies in the Coma cluster from Mobasher et al. (2001, Paper II of the series). This catalog comprises ∼300 spectra of cluster members with absolute magnitudes in the range M B = −20.5 to −14 in two areas of ∼ 1 × 1.5 Mpc towards the center and the South-West 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 λ = 3600 − 6600Å.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 Hβ, Hγ F and Hδ 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>, 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 South-West 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, > 30% of the dwarfs) show no evidence in their central regions of star formation occurred 1 Based on observations made with the William Herschel Telescope operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos

The Next Generation Virgo Cluster Survey (NGVS). XXIV. The Red Sequence to ∼106 L⊙ and Comparisons with Galaxy Formation Models

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Quantifying the Bimodal Color‐Magnitude Distribution of Galaxies

“…In other words, if there has been on average more recent star formation in a class of galaxies, then their mean color becomes bluer and the color dispersion increases for any reasonable variation in their precise SFHs. However, we cannot rule out the transition also being caused by a metallicityluminosity correlation as long as the metallicity dispersion increases with decreasing galaxy luminosity (Poggianti et al 2001).…”

Globular cluster systems of six shell galaxies

“…First, galaxies are clearly not SSPs, and merger remnants in particular will have at least three episodes of star formation, corresponding to the two progenitors and to the merger induced star formation event. Second, the degeneracy between age and metallicity, combined with uncertainty in isochrone models, and assumptions made about other parameters of the stellar population, such as the mass function, render SSP ages very uncertain (Poggianti et al 2001). Third SSP ages are derived from nuclear spectra only, which can be unrepresentative of the galaxy as a whole (e.g.…”