• methods: statistical;
  • galaxies: clusters: general;
  • galaxies: clusters: individual: RzCS 052;
  • galaxies: evolution;
  • dark matter;
  • X-rays: galaxies: clusters


We report on the discovery of the z= 1.016 cluster RzCS 052 using a modified red-sequence method, follow up spectroscopy and X-ray imaging. This cluster has a velocity dispersion of 710 ± 150 km s−1, a virial mass of 4.0 × 1014 M (based on 21 spectroscopically confirmed members) and an X-ray luminosity of (0.68 ± 0.47) × 1044 erg s−1 in the [1–4] keV band. This optically selected cluster appears to be of richness class 3 and to follow the known LX–σv relation for high-redshift X-ray selected clusters. Using these data, we find that the halo occupation number for this cluster is only marginally consistent with what was expected assuming a self-similar evolution of cluster scaling relations, suggesting perhaps a break of them at z∼ 1. We also rule out a strong galaxy merging activity between z= 1 and today. Finally, we present a Bayesian approach to measuring cluster velocity dispersions and X-ray luminosities in the presence of a background: we critically reanalyse recent claims for X-ray underluminous clusters using these techniques and find that the clusters can be accommodated within the existing LX–σv relation.