PHOTOMETRIC PROPERTIES OF CLUSTERS OF GALAXIES

We report the first results of a long-term program aimed at investigat ing the photometric properties of the cores of Abell and X-ray-selecte d clusters of galaxies from the Extended Medium Sensitivity Survey pub lished by Gioia et al. in 1990. We observed 77 clusters of galaxies in the redshift range 0.05 less than or equal to z less than or equal to 0.25 in the Gunn g, r, and i filters: in this paper, we present the d ata on 59 clusters with good absolute photometry and on another group of eight clusters with acceptable relative photometry. For all these c lusters, we show color-magnitude diagrams in the two colors:. when, as in most cases, the early-type galaxy sequence is identifiable, we com pare it with the expectation from the Virgo C-M relation as presented by Visvanathan & Sandage in 1977 and find that the Virgo relation hold s to z similar to 0.2. We do not find any sign of active evolution in cluster galaxies since that epoch or in the percentage of blue galaxie s 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 b asis 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 r edshift higher than the cluster one. Finally, we briefly compare a few properties of the two subsamples of optically selected and X-ray-sele cted clusters.