Stellar populations of early-type galaxies in different environments III. Line-strength gradients

Abstract

We present line-strength gradients for 22 spectral indices measured in a sample of 82 early-type galaxies in different environments,including the high-density core of the Coma cluster, the Virgo cluster,poor groups,and field galaxies. We derive age and metallicity gradients, and compare the mean values with the predictions of different galaxy formation models. We explore the behaviour of individual chemical species by deriving the metallicity gradient with different indicators.We find that the strength of the metallicity gradient inferred from stellar population models depends on the specific Lick index employed. In particular, metallicity gradients obtained with CN2 and C4668 combined with Hb are steeper than when measured using Ca4227 or Fe4383. The correlation of the metallicity gradients with other parameters also depends on the specific index employed. If the metallicity gradient is obtained using CN2 and Mgb then it correlates with the central age of the galaxies. On the contrary, if Fe4383 or Ca4227 are used, the metallicity gradient correlates with the velocity dispersion gradient.This may suggests that several mechanism have helped to set the age and metallicity gradients in early-type galaxies. While we do not find any correlation between the metallicity gradient and the central velocity dispersion for galaxies in low-density environments, we find a marginal correlation between the metallicity gradient and the mass for galaxies in the centre of the Coma cluster. We also find a trend for which galaxies in denser environments show a steeper metallicity gradient than galaxies in less dense environments.We interpret these results in light of the different models to explain the differences between galaxies as a function of environment.