Authors: Mark den Brok, R.F. Peletier, E.A. Valentyn, M. Balcells, D. Carter, P. Erwin, H.C. Ferguson, P. Goudfrooij, A.W. Graham, D. Hammer, J.R. Lucey, N. Trentham, R. Guzman, C. Hoyos, G. Verdoes Kleijn, S. Jogee, A.M. Karick, I. Marinova, M. Mouhcine, and T. Weinzirl
Abstract: We use very deep data of the HST ACS Coma Cluster Treasury Survey to determine colour gradients of a large sample of early-type galaxies in the Coma cluster. The Coma cluster is one of the nearest rich galaxy clusters. Since extinction in early- type Coma galaxies is negligible in general, colour gradients can be interpreted as stellar population gradients which provide us information about galaxy formation mechanisms. Given its large fraction of early-type galaxies and lack of cold ISM, there has probably been much less recent star formation in Coma than in other nearby clusters or in the field. For that reason it is probably the best place in the local Universe to study metallicity gradients. This paper shows that colour gradients are small and negative, and decreasing from giants to dwarfs. Our sample consists of ~140 galaxies brighter than F814W(AB) < -15 mag, which were either spectroscopically confirmed members of Coma or identified by eye. If we do not consider the nuclear star clusters in the galaxies, the colour gradients that we obtain are almost all negative and show a reasonably large intrinsic scatter for elliptical and lenticular galaxies. The gradients of dwarf galaxies form a continuous sequence with those of elliptical galaxies but become shallower for fainter magnitudes. In contrast with earlier claims that dwarf galaxies have positive gradients, we show that the gradients tend to become flat, but not positive, meaning that early-type galaxies in the Coma cluster are probably less metal rich in the outer parts. Our analysis underlines that high resolution data are very important to separate different morphological components in galaxies. Here we do not find any evidence for environmental influence on the gradients, but this not a strong conclusion, since most of our galaxies are found in the central regions of the cluster. For a subset of galaxies of which the morphologies are known, we find that S0 galaxies have less steep gradients than elliptical galaxies.