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

Mean ages and metal abundances are estimated for the stellar populations in a sample of 115 E and S0 galaxies in the central 64'x70' of the Coma cluster. The estimates are based on the line indices Mg2, and Hbeta, and the mass-to-light ratios (M/L). Stellar population models from Vazdekis et al. were used to transform from the measured parameters to mean ages, Mg/H and Fe/H. Comparison of the ages derived from the Mg2-Hbeta diagram to those derived from the Mg2-M/L diagram gives an estimate of the variation in the fraction of dark matter. The distributions of the derived mean ages and abundances show that there are real variations in both the mean ages and in the abundances. We find an intrinsic rms scatter of Mg/H, Fe/H and Mg/Fe of 0.2 dex, and an intrinsic rms scatter of the derived ages of 0.17 dex. The slopes of the scaling relations between the global parameters for the galaxies (the Mg2-sigma relation, the -sigma relation, the Hbeta-sigma relation) are consistent with the relation we find between the ages, the abundances and the velocity dispersions. The slope of the Fundamental Plane is steeper than predicted from the variations in the ages and abundances. Because of the correlation between the mean ages and the mean abundances, substantial variations in the ages and the abundances are possible while maintaining a low scatter of all the scaling relations.