2004ApJ...616..192C

We examine whether early-type galaxies in clusters may have evolved from later types by the fading of their disks (e.g., as a result of ram pressure stripping or strangulation) or by enhancement of the bulge luminosity (e.g., due to tidal interactions and mergers). For this purpose, we compare the bulge and disk luminosities of early- and late-type galaxies and of galaxies at different radial distances from the cluster center. We find that in order for early-type galaxies, including S0, to have evolved from late-type galaxies, their bulge luminosities must have been physically enhanced. Disk-fading models cannot explain the differences observed. We then show that galaxy bulges are systematically brighter at small projected distances from the cluster center, while disk luminosities are uncorrelated with clustercentric distance. Our results suggest that bulge enhancement, not disk fading, distinguishes early- from late-type galaxies and is thus at least partially responsible for the morphology-environment relation of bright cluster galaxies.