AAS Meeting #193 - Austin, Texas, January 1999
Session 38. Galaxy Clusters
Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibit Hall 1

## [38.16] Where are all the Butcher-Oemler Galaxies Now?

D. H. McIntosh, H.W. Rix (Steward Obs.), N. Caldwell (SAO)

Numerous studies have established the Butcher-Oemler effect, which is the observed increase of blue cluster galaxies with increasing redshift out to z~~1. Yet the cause of this rapid evolution and the fate of the blue galaxies is not understood. We have begun a study to test the hypothesis that the Butcher-Oemler effect is mostly attributable to spiral disk galaxies, whose star formation ceased between z=1 and z=0, turning them into S0 galaxies. The resulting S0's should now be abundant in nearby clusters and have intermediate age populations in their disks. While considerable effort has gone into searching for really young (E+A,1Gyr) galaxies in local clusters, no systematic, large-scale search for these intermediate-aged S0 galaxies in nearby clusters has been conducted. Thus we have obtained UVI photometry for a sample of 10 local (z\leq 0.06) Abell clusters, most with more than 100 spectroscopically confirmed members.

We have used wide-field'' format CCDs on the Whipple Observatory 48-inch and on the KPNO 0.9-meter to achieve 1\arcdeg \times 1\arcdeg coverage (~-4~Mpc per axis) of each cluster in all three colors. We will use the resultant colors and galaxy morphology to select non-star-forming early-type (E/S0's) galaxies from each cluster. In addition, we can select out late-type galaxies currently forming stars by their H\alpha emission (from the extant spectra). While old (\geq~10~Gyr) E/S0's in the cores of dense clusters obey a tight color-magnitude relation, galaxies with an intermediate age population ( e.g. ~ 4~Gyr corresponds to z\rm cessation=0.3) will appear significantly bluer in their U-V colors. For instance, population synthesis modeling predicts \Delta(U-V)=0.14 between a 4~Gyr and 8~Gyr population. Once identified, these younger E/S0's will be fit with two-dimensional bulge/disk models to test whether the younger stellar populations are preferably found in S0's, and if so, whether they reside within their disks rather than their bulges. Further, we will determine if these younger S0's preferentially avoid the parent cluster core.

This data will be used in conjunction with archival HST WFPC2 images of z~.4 clusters (i.e. comparable spatial resolutions) to investigate key issues of cluster galaxy evolution such as: have the bulges of cluster disks grown over time?