Internal colour gradients for E/S0 galaxies in Abell 2218

Abstract

ABSTRACT We determine colour gradients of −0.15±0.08 magnitudes per decade in radius inF450W−F606Wand −0.07±0.06 magnitudes per decade in radius in F606W−F814Wfor a sample of 22 E/S0 galaxies in Abell 2218. These gradients are consistent with theexistence of a mild (∼ −0.3 dex per decade in radius) gradient in metal abundance,(cf. previous work at lower and higher redshift for field and cluster galaxies). Thesize of the observed gradients is found to be independent of luminosity over a rangespanning M ∗ −1 to M ∗ +1.5 and also to be independent of morphological type. Theseresults suggest a fundamental similarity in the distributions of stellar populations inellipticals and the bulges of lenticular galaxies. These results are not consistent withsimple models of either monolithic collapse or hierarchical mergers.Key words: galaxies:clusters — galaxies: formation and evolution 1 INTRODUCTIONIn Hubble’s (1936) classification scheme for galaxies, lentic-ular (S0) galaxies first appear as a transitional class betweenthe ellipticals and the two branches (barred and unbarred)of the spirals, and, despite much evidence to the contrary,it has often been assumed that the Hubble ‘fork’ describesan evolutionary scheme. For instance, this assumption in-forms the popular idea that collisions of proto-disks in theearly universe lead to the formation of elliptical galaxiesor that quenching of star formation in field spirals as theyfall into dense regions leads, by a variety of mechanisms,to their transformation into S0 galaxies. This process, firstproposed by Larson, Tinsley & Caldwell (1980), has beeninvoked to account for the morphology-density relation ofDressler (1980) and the apparent blueing of the galaxy pop-ulation in distant clusters (Butcher & Oemler 1984), andhas stimulated considerable theoretical activity (see Pimb-blet 2003 for a review).Such processes modify the star formation history of thedisk by removing the supplies of gas necessary to fuel furtherstar formation. A sensitive test of this mechanism wouldtherefore involve a comparison of the stellar populations ofbulges and disks for elliptical, S0 and spiral galaxies. Whilethe stellar content of galaxies cannot be resolved except for