Galaxy evolution in clusters up to z = 1 . 0 ⋆

We present a combined study of the colour–magnitude relation, colour distribution and luminosity function of a sample of 24 clusters at redshifts 0.3 < z < 1. The sample is largely composed of X–ray selected/detected clusters. Most of the clusters at redshifts z < 0.6 display X–ray luminosity or richness typical of poor clusters or groups, rather than the more typical, massive clusters studied in literature at redshifts z 0.3. All our clusters, including groups, display a colour–magnitude relation consistent with a passively evolving stellar population formed at a redshift z f 2, in accordance with observed galaxy populations in more massive clusters studied at comparable redshifts. Colours and luminosity functions show that the cluster galaxy population is consistent with the presence of at least two components: old systems formed at high redshift that have evolved passively from that epoch, together with a galaxy population displaying more recent star formation. The former population forms at 2 z f 5, the latter at redshifts z < 1. A model in which stars do not evolve is clearly rejected by both by the colour of reddest galaxies and by the characteristic luminosity m * measures. All clusters (with one possible exception) are detected independently by an almost three dimensional optical search employing sky position and colour – this despite the primary X–ray selection and low X–ray flux/optical richness displayed by the majority of the sample.