Stellar populations of early-type galaxies in different environments

III. Line-strength gradients

¹ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPF L), Observatoire, 1290 Sauverny, Switzerland e-mail: Patricia.SanchezBlazquez@epfl.ch
² Dpto. de Astrofísica, Fac. de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
³ Calar Alto Observatory, CAHA, Apartado 511, 04004 Almería, Spain

Received: 11 January 2006
Accepted: 19 April 2006

Abstract

Aims.This is the third paper of a series devoted to the study of the stellar content of early-type galaxies. The goal of the series is to set constraints on the evolutionary status of these objects.

Methods.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 cores of the Coma cluster, the Virgo cluster, poor groups, and isolated field galaxies. Using new evolutionary population synthesis models, 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.

Results.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 Hβ are steeper than those 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 is used, the metallicity gradient correlates with the velocity dispersion gradient.

Conclusions.This may suggest that several mechanisms 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 mechanisms proposed to explain the observed changes between galaxies as a function of environment.