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

Abstract

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. using the Argus 24-arm positioner and spectrograph, which is a facility instrument on the CTIO 4m telescope. Input coordinates for the galaxies in these clusters were measured using either the Grant measuring engine at CTIO, or the one at KPNO, using the Dressler (1980) coordinates or those of Malumuth et al. (1992) as first estimates. The coordinates reported in the table below are accurate to about 1.5˝. Observations were limited to the early-type galaxies as classified by , except as noted in section 3 below. Because of the density of targets in these clusters, several setups were required per field observed. Statistics on the numbers of galaxies observed relative to the number of candidates appear below.The fibers fed a bench mounted spectrograph, whose detector was a Reticon 1200X400 CCD. A blue-blazed grating was used that gave a resolution of 2 pixels or 3.6Å, and a wavelength coverage from 3500 to 5700Å. Exposure times for ...

Wings-Spe

“…This number is in strong contrast with the value found from the catalogue of local clusters of Dressler & Shectman (1988) but is broadly consistent with a fraction of ∼15% found among lower luminosity galaxies in 5 low-z cluster by Caldwell & Rose (1997). The reason for the striking difference with the sample of Dressler & Shectman (1988) is probably due to the difference in the magnitude limit of the two surveys.…”

K+a galaxies in the zCOSMOS survey

“…In some studies, post-starburst galaxies are found to be mostly bulge-dominated and/or have a morphology that is consistent with being early-type (Goto & SDSS collaboration 2003;Quintero et al 2004;Tran et al 2004;Blake et al 2004;Goto 2005;Balogh et al 2005;Poggianti et al 2009), and in some others have a high incidence of disks and spiral arms (e.g., Franx 1993;Couch et al 1994;Dressler et al 1994;Caldwell & Rose 1997;Dressler et al 1999). For example, Quintero et al (2004) visually inspected a sample of 160 galaxies from the SDSS at 0.05 < z < 0.20, and classified 90% of them as bulge-dominated systems.…”

Velocity segregation effects in galaxy clusters at 0.4 ≲z≲ 1.5

“…For instance, the relation between the VCS and cluster dynamical status has been explored in very few studies (Ribeiro et al 2010(Ribeiro et al , 2013. The member selection might be particularly critical in very active clusters, and the scenario is made more complex by the fact that cluster mergers might also enhance star formation in galaxies (e.g., Caldwell & Rose 1997;Ferrari et al 2005;Owen et al 2005). The dependence of VCS on redshift is also only rarely investigated.…”