Photometric Properties of Clusters of Galaxies

We report the first results of a long-term program aimed at investigating the photometric properties of the cores of Abell and X-ray- selected clusters of galaxies from the Extended Medium Sensitivity Survey published by Gioia et al. in 1990. We observed 77 clusters of galaxies in the redshift range 0.05 <= z <= 0.25 in the Gunn g, r, and i filters: in this paper, we present the data on 59 clusters with good absolute photometry and on another group of eight clusters with acceptable relative photometry. For all these clusters, we show color-magnitude diagrams in the two colors: when, as in most cases, the early-type galaxy sequence is identifiable, we compare it with the expectation from the Virgo C-M relation as presented by Visvanathan & Sandage in 1977 and find that the Virgo relation holds to z ~ 0.2. We do not find any sign of active evolution in cluster galaxies since that epoch or in the percentage of blue galaxies or in the early-type galaxy colors, if we accept that the scatter is of the order of 0.2-0.3 mag with respect to the expectations on the basis of standard K-corrections. We point out the presence of a certain number of anomalously red galaxies in the r - i color, which are too red with respect to their g - r color to be normal field galaxies at a redshift higher than the cluster one. Finally, we briefly compare a few properties of the two subsamples of optically selected and X-ray-selected clusters.