Photometric properties of clusters of galaxies

Abstract

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 (EMSS, \cite{emss}) clusters of galaxies. We observed 77 clusters of galaxies in the redshift range $0.05\leq z \leq 0.25$ 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 8 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 (\cite{vs}) and find that the Virgo relation holds to $z\sim0.2$. We do not find any sign of active evolution in cluster galaxies since that epoch, nor 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.