Spatially resolved spectroscopy of Coma cluster early-type galaxies - III. The stellar population gradients

Based on Paper I of this series (Mehlert et al. 2000), we derive central values and logarithmic gradients for the H (beta), Mg and Fe indices of 35 early-type galaxies in the Coma cluster. We find that pure elliptical galaxies have on average slightly higher velocity dispersions, lower H (beta), and higher metallic line-strengths than galaxies with disks (S0). The latter form two families, one comparable to the ellipticals and a second one with significantly higher H (beta), and weaker metallic lines. Our measured logarithmic gradients within the effective radius are .... The gradients strongly correlate with the gradients of (sigma) , but only weakly with the central index values and galaxy velocity dispersion. Using stellar population models with variable element abundance ratios from Thomas et al. (2003a) we derive average ages, metallicities and [ (alpha)/Fe ]in the center and at the effective radius. We find that the (alpha)/Fe ratio correlates with velocity dispersion and drives 30% of the Mg- (sigma) relation, the remaining 70% being caused by metallicity variations. We confirm previous findings that part of the lenticular galaxies in the Coma cluster host very young ( local relation inside a galaxy, unlike the global Mg- (sigma) relation, must be primarily driven by metallicity variations alone. Finally we note that none of the stellar population parameters or their gradients depend on the density profile of the Coma cluster, even though it spans 3 dex in density.