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

Abstract

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 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, Maraston & Bender (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 derive negative metallicity gradients (-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. 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 the mean. 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.