The population of galaxies in the distant clusters Cl 1613+3104 and Cl 1600+4109

Abstract

We study the photometric properties and content of the distant clusters of galaxies Cl 1613+3104 (z = 0.415) and Cl 1600+4109 (z = 0.540). The former is a rich, concentrated cluster which shows a strong evidence for segregation in luminosity and color. Its population of galaxies separates into two different kinds of objects: a red population compatible with the colors expected for E/S0 galaxies located preferentially in the central part of the cluster, and a sparse blue population explained in part by the presence of normal S/Im galaxies, which is much less concentrated. In the case of Cl 1600+4109, there is a lack of E/S0 galaxies and the population is mainly made up of S/Im galaxies, without any evidence of segregation in magnitude or in color. Both clusters exhibit an important fraction of blue objects, increasing with magnitude. This result is due in part to the presence of normal S/Im galaxies, and also to extremely blue objects probably undergoing an episode of intense star-formation.