Indications for 3 Mpc-scale large-scale structure associated with an X-ray luminous cluster of galaxies at z=0.95 ?

Context. X-ray luminous clusters of galaxies at z ∼ 1 are emerging as major cosmological probes and are fundamental tools to study the cosmic large-scale structure and environmental effects of galaxy evolution at large look-back times. Aims. We present details of the newly-discovered galaxy cluster XMMU J0104.4-0630 at z = 0.947 and a probable associated system in the LSS environment. Methods. The clusters were found in a systematic study for high-redshift systems using deep archival XMM-Newton data for the serendipitous detection and the X-ray analysis, complemented by optical/near-infrared (NIR) imaging observations and spectroscopy of the main cluster. Results. We find a well-evolved, intermediate luminosity cluster with L 0.5−2. 0k eV X = (6.4 ± 1.3) × 10 43 h −2 70 erg s −1 and strong central 1.4 GHz radio emission. The cluster galaxy population exhibits a pronounced transition toward bluer colors at cluster-centric distances of 1−2 core radii, consistent with an age difference of 1−2 Gyr for a single burst solar metallicity model. The second, less evolved X-ray cluster at a projected distance of 6.4 � (∼3 Mpc) and a concordant red-sequence color likely forms a cluster-cluster bridge with the main target as part of its surrounding large-scale structure at z � 0.95.