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 Hbeta, 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 Hbeta, 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 Hbeta, and weaker metallic lines. Our measured logarithmic gradients within the effective radius are [DeltaMg b] approximate to -0.037, [Delta[Fe]] approximate to -0.029, [DeltaHbeta] approximate to + 0.017 and [Deltasigma] approximate to -0.063. 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] ratios 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 (similar to2 Gyr) stellar populations, hence must have experienced relatively recent star formation episodes. Again in accordance with previous work we derive negative metallicity gradients (similar to-0.16 dex per decade) that are significantly flatter than what is expected from gaseous monolithic collapse models, pointing to the importance of mergers in the galaxy formation history. Moreover, the metallicity gradients correlate with the velocity dispersion gradients, confirming empirically earlier suggestions that the metallicity gradient in ellipticals is produced by the local potential well. The gradients in age are negligible, implying that no significant residual star formation has occurred either in the center or in the outer parts of the galaxies, and that the stellar populations at different radii must have formed at a common epoch. For the first time we derive the gradients of the alpha/Fe ratio and find them very small on average. Hence, alpha/Fe enhancement is not restricted to galaxy centers but it is a global phenomenon. Our results imply that the Mg-sigma 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.