The Butcher-Oemler Effect in Nearby Clusters

Abstract

Probably the most striking evidence for galaxy evolution at recent epochs has been the discovery of a rapid change in the nature of galaxy populations in clusters over the redshift range z=0 to z=0.5. The ‘classical’ Butcher-Oemler effect (Butcher & Oemler 1978, 1984) used photomteric studies to reveal an unexpected increase in the fraction of blue galaxies in the cores of distant (z ~ 0.4) rich concentrated clusters when compared with nearby (z< 0.05) clusters of similarly high richness and central concentration. An alternative view, based on spectroscopic studies (Dressler & Gunn 1982; Couch & Sharpies 1987), manifests itself as an increase in the fraction of active galaxies which show signs of recent star formation and/or nuclear activity. Some of these galaxies are indeed blue but some (e.g. in C10016+16, Dressler & Gunn 1992) are red. Although it is the very absence of blue galaxies in nearby clusters which defines the classical Butcher-Oemler effect, comparable spectroscopic studies of nearby cluster populations with the appropriate completeness and high signal-to-noise required for population (as opposed to dynamical) studies have only recently been undertaken. In at least one case these have revealed unexpected similarities to the spectroscopic signatures which appear so prevalent at higher redshifts.