SIMBAD references

We present gri CCD photometry of 44 Abell clusters and four 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 approximately 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, with an increase in the fraction of blue galaxies (f_B) with redshift that seems more consistent with the steeper relation estimated in 1995 by Rakos and Schombert than with the original one by Butcher and Oemler in 1984. However, in the redshift range between about 0.08 and 0.2, where most of our clusters lie, there is a wide range of f_B-values, consistent with no redshift evolution of the cluster galaxy population. A large range of f_B-values is also seen between about 0.2 and 0.3, when we add X-ray clusters from Smail et al. 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. in 1996 that there is a population of unusually red galaxies that 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.