Stellar population gradients in early-type cluster galaxies

Abstract

We present a study of internal stellar population gradients in early-type cluster galaxies. Using the Very Large Telescope (VLT) Visible Multi-Object Spectrograph (VIMOS) integral field unit, we observed 19 galaxies in the core of the Shapley supercluster (z = 0.048). The radial trends in nine absorption lines (HδF to Fe5406) were measured to the effective radius for 14 galaxies, from which we derived the gradients in age, total metallicity and α-element overabundance. We combine these with results from 11 galaxies studied in our previous VIMOS work. We observe a mean metallicity gradient of -0.13 ± 0.04 dex-1 and, in common with the findings of previous studies, galaxies with log σ ≳ 2.1 have a sizeable intrinsic scatter in metallicity gradient. The mean log(age/Gyr) gradient is -0.02 ± 0.06 dex-1, although several galaxies have significant positive or negative age gradients. The mean gradient in α-element enhancement is -0.10 ± 0.04 dex-1. We find that stellar population gradients are primarily related to the central metallicity: early-type galaxies with supersolar centres have steep negative metallicity gradients and positive age gradients; those with solar metallicity centres have negligible [Z/H] gradients and negative age gradients. There is a strong observed anticorrelation between the gradients in age and metallicity. While a part of this trend can be attributed to the correlation of measurement errors, we demonstrate that there is an underlying intrinsic relation. For the Shapley galaxies, B - R colour gradients predicted from spectroscopic age and metallicity generally agree well with those measured directly from photometry. © 2009 RAS.