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 > 10¹⁴ M_⊙ 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^{bol_X,500≃(2.1 ± 0.4)× 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 9.4 Gyr, 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.

Based on observations under programme ID 081.A-0312 collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, and observations collected at the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto, operated jointly by the Max-Planck Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC).Figure 2 and Tables 1 and 2 are only available in electronic form at http://www.aanda.org