The Butcher-Oemler Effect at Low Redshift: Spectroscopy of Five Nearby Clusters of Galaxies

We present multi-fiber spectroscopy and broadband imaging of early-type galaxies in five nearby rich clusters of galaxies. The main purpose was to look for “abnormal” spectrum galaxies (i.e., post-starburst galaxies which have strong Balmer absorption lines and emission line galaxies) in nearby clusters that are similar to those found by Caldwell et al. (1993) in the Coma cluster. Three of the clusters were purposefully selected to have pronounced double structure in their spatial distribution of the galaxies. Our primary conclusion is that ∼15% of the early-type galaxies in these nearby rich clusters have signs of ongoing or recent star formation. Furthermore, the starburst and post-starburst nature of these abnormal spectra is reminiscent of that seen in distant clusters, although at a reduced frequency and burst strength. Thus activity similar to that seen in distant clusters is still ongoing, at a reduced level, in present-epoch rich clusters. The frequency of such galaxies appears to be enhanced significantly over that seen in field galaxies. Most of the new post-starburst galaxies are disk galaxies; three E galaxies in one cluster and one in another cluster have current star formation. We also find evidence in the spatial and kinematic structure of several of the clusters that subclusters have recently passed through the main clusters and are now emerging out the other side. This inference is based on a comparison of the spatial and kinematic cluster data with N-body simulations of infalling clusters. Specifically, we see evidence of velocity gradients and/or a dispersed appearance of the observed subclusters, both of which are produced by tidal distension and disruption of infalling subclusters. If, as we suspect, the subclusters passed through the main clusters more than a Gyr ago, then the post-starburst timescales of ∼1 Gyr imply that the star formation bursts are only triggered during (or after) the subcluster passage through the center of the main cluster. We speculate that shocks induced in the collisions of the cluster and subcluster ICM’s may trigger the galaxy starbursts.