Letters to the Editor

The X-ray luminous galaxy cluster XMMU J1007.4+1237 at z = 1.56^⋆,⋆⋆

The dawn of starburst activity in cluster cores

¹ Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany
e-mail: rfassben@mpe.mpg.de
² INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
³ European Southern Observatory (ESO), Karl-Scharzschild-Str. 2, 85748 Garching, Germany
⁴ Departamento de Astronomía y Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁵ Astrophysikalisches Institut Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ University Observatory Munich, Ludwigs-Maximillians University Munich, Scheinerstr. 1, 81679 Munich, Germany
⁷ CEASaclay, Service d’Astrophysique, L’Orme des Merisiers, Bât. 709, 91191 Gif-sur-Yvette Cedex, France
⁸ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany

Received: 18 November 2010
Accepted: 4 January 2011

Abstract

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  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.