A statistical analysis of the galaxy populations of distant luminous X-ray clusters

Abstract

We present a deep, multicolour (UBI) CCD survey using the Palomar 5-m telescope of a sample of high X-ray luminosity, distant clusters selected from the ROSAT All-Sky Survey. The 10 clusters lie in the redshift range z= 0.22–0.28, an era in which evolutionary effects have been reported in the properties of cluster galaxy populations.Our clusters thus provide a well-defined sample of the most massive systems at these redshifts with which to quantify the extent and variability of these evolutionary effects. The relatively low redshifts of these clusters also means that simple connections can be made between the galaxy populations of these clusters and their immediate descendents, local rich clusters. Moreover, by concentrating on a narrow redshift range, we can take advantage of the homogeneity of our cluster sample to combine the galaxy catalogues from all the clusters to analyse statistically the bulk properties of their populations. We present an analysis of the cluster galaxy populations using our multicolour data to probe the distribution, luminosities and star formation histories of galaxies in these regions. Our aim is to chart the characteristics of the galaxy populations of massive intermediate redshift clusters and to combine these into a wider scheme for galaxy evolution in high-density environments. The core regions of clusters in our sample contain only a small proportion of star-forming galaxies, and they therefore do not exhibit a classical “Butcher—Oemler” effect. Focusing on the redder cluster galaxies, we find that their integrated luminosity is well correlated with the X-ray temperatures of the clusters, and hence with cluster mass. Furthermore, the typical rest frame UV—optical colours of the luminous elliptical sequences in the clusters exhibit a remarkably small cluster-to-cluster scatter, ≲2 per cent, indicating that these galaxies are highly homogeneous between cluster environments. However, at fainter magnitudes we observe a marked increase in the range of mid-UV colours of galaxies possessing strong 4000-Å breaks, as determined from photometry in ∼7.5 h-1 kpc diameter apertures. In the light of the apparent decline in the population of S0 galaxies seen in distant z≳0.4, clusters, and in view of the luminosities and colours of this population, we propose that they may be the progenitors of the dominant S0 population of rich local clusters, caught in the final stage before they become completely quiescent. Further studies of this population will prove a necessary link with which to connect the evolution observed in cluster populations at high redshift with the nature of the final remnants locally. Observations in the rest frame UV will be important in these studies owing to the relative ease of detecting the signature of previous activity in this spectral region.