Stellar population gradients in early - type cluster galaxies ⋆

We present a study of internal stellar population gradients in early-type cluster galaxies. Using the VLT 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 over-abundance. We combine these with results from 11 galaxies studied in our previous VIMOS work (Rawle et al. 2008b). We observe a mean metallicity gradient of –0.13 ± 0.04 dex 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, although several galaxies have significant positive or negative age gradients. The mean gradient in α-element enhancement is –0.10 ± 0.04 dex. We find that stellar population gradients are primarily related to the central metallicity: early-type galaxies with super-solar 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.