The X-ray luminous galaxy cluster XMMU J1007.4+1237 at z=1.56 The dawn of starburst activity in cluster cores

Context. Observational galaxy cluster studies at z > 1.5 probe the formation of the first massive M > 10(14) M-circle dot dark matter halos, the early thermal history of the hot ICM, and the emergence of the red-sequence population of quenched early-type galaxies. Aims. We present first results for the newly discovered X-ray luminous galaxy cluster XMMU J1007.4+1237 at z = 1.555, detected and confirmed by the XMM-Newton Distant Cluster Project (XDCP) survey. Methods. We selected the system as a serendipitous weak extended X-ray source in XMM-Newton archival data and followed it up with two-band near-infrared imaging and deep optical spectroscopy. Results. We can establish XMMU J1007.4+1237 as a spectroscopically confirmed, massive, bona fide galaxy cluster with a bolometric X-ray luminosity of L-X,500(bol) similar or equal to (2.1 +/- 0.4) x 10(44) erg/s, a red galaxy population centered on the X-ray emission, and a central radio-loud brightest cluster galaxy. However, we see evidence for the first time that the massive end of the galaxy population and the cluster red-sequence are not yet fully in place. In particular, we find ongoing starburst activity for the third ranked galaxy close to the center and another slightly fainter object. Conclusions. At a lookback time of 9.4Gyr, the cluster galaxy population appears to be caught in an important evolutionary phase, prior to full star-formation quenching and mass assembly in the core region. X-ray selection techniques are an efficient means of identifying and probing the most distant clusters without any prior assumptions about their galaxy content.