Photometric Properties of 48 Clusters of Galaxies. I. The Butcher-Oemler Effect

We present gri CCD photometry of 44 Abell clusters and 4 cluster candidates. Twenty one clusters in our sample have spectroscopic redshifts. Fitting a relation between mean g, r and i magnitudes, and redshift for this subsample, we have calculated photometric redshifts for the remainder with an estimated accuracy of ∼ 0.03. The resulting redshift range for the sample is 0.03 z 0.38. Color-magnitude diagrams are presented for the complete sample and used to study evolution of the galaxy population in the cluster environment. Our observations show a strong Butcher-Oemler effect (Butcher Oemler 1978, 1984), with an increase in the fraction of blue galaxies (fB) with redshift that seems more consistent with the steeper relation estimated by Rakos and Schombert (1995) than with the original one by Butcher Oemler (1984). However, in the redshift range between ∼ 0.08 and 0.2, where most of our clusters lie, there is a wide range of fB values, consistent with no redshift evolution of the cluster galaxy population. A large range of fB values is also seen between ∼ 0.2 and 0.3, when Smail at al. (1998) x-ray clusters are added to our sample. The discrepancies between samples underscore the need for an unbiased sample to understand how much of the Butcher-Oemler effect is due to evolution, and how much to selection effects. We also tested the idea proposed by Garilli et al. (1996) that there is a population of unusually red galaxies which could be associated either with the field or clusters, but we find that these objects are all near the limiting magnitude of the images (20.5 r 22) and have colors that are consistent with those expected for stars or field galaxies at z ∼ 0.7. Subject headings: galaxies: clusters – evolution – color – photometry