E and S0 galaxies in the central part of the Coma cluster: ages, metal abundances and dark matter

Abstract

Mean ages and metal abundances are estimated for the stellar populations in a\nsample of 115 E and S0 galaxies in the central 64'x70' of the Coma cluster. The\nestimates are based on the line indices Mg2, <Fe> and Hbeta, and the\nmass-to-light ratios (M/L). Stellar population models from Vazdekis et al. were\nused to transform from the measured parameters to mean ages, [Mg/H] and [Fe/H].\nComparison of the ages derived from the Mg2-Hbeta diagram to those derived from\nthe Mg2-M/L diagram gives an estimate of the variation in the fraction of dark\nmatter. The distributions of the derived mean ages and abundances show that\nthere are real variations in both the mean ages and in the abundances. We find\nan intrinsic rms scatter of [Mg/H], [Fe/H] and [Mg/Fe] of 0.2 dex, and an\nintrinsic rms scatter of the derived ages of 0.17 dex.\n The slopes of the scaling relations between the global parameters for the\ngalaxies (the Mg2-sigma relation, the <Fe>-sigma relation, the Hbeta-sigma\nrelation) are consistent with the relation we find between the ages, the\nabundances and the velocity dispersions. The slope of the Fundamental Plane is\nsteeper than predicted from the variations in the ages and abundances. Because\nof the correlation between the mean ages and the mean abundances, substantial\nvariations in the ages and the abundances are possible while maintaining a low\nscatter of all the scaling relations.\n