First simultaneous optical/near-infrared imaging of an X-ray selected, high-redshift cluster of galaxies with GROND The galaxy population of XMMU J0338.7+0030 at z=1.1

Context. The XMM-Newton Distant Cluster Project is a serendipitous survey for clusters of galaxies at redshifts z >= 0.8 based on deep archival XMM-Newton observations. X-ray sources identified as extended are screened against existing optical all-sky surveys for galaxies, in case of candidate high-z clusters followed up with imaging at 4 m-class telescopes and, ultimately, multi-object spectroscopy at 8 m-class telescopes. Low-significance candidate high-z clusters are followed up with the seven-channel imager GROND (Gamma-Ray Burst Optical and Near-Infrared Detector) that is mounted at a 2 m-class telescope. Its unique capability of simultaneous imaging in the g', r', i', z', J, H, Ks bands enables the use of the photometric redshift technique. Aims. Observing strategy, data reduction and analysis, depth and accuracy of the simultaneous multi-wavelength photometry are discussed with the goal of establishing GROND as a useful instrument to confirm X-ray selected (high-z) clusters. Methods. The test case is XMMUJ0338.7+0030, suggested to be at z similar to 1.45 +/- 0.15 (1 sigma) from the analysis of the z - H vs. H colour-magnitude diagram obtained from the follow-up imaging. Later VLT-FORS2 spectroscopy enabled us to identify four members, which set this cluster at z = 1.097 +/- 0.002 (1 sigma). To reach a better knowledge of its galaxy population, we observed XMMUJ0338.7+0030 with GROND for about 6 h. The publicly available photo-z code le Phare was used. Results. The Ks-band number counts of the non-stellar sources out of the 832 detected down to z' similar to 26 AB mag (1 sigma) in the 3.9x4.3 arcmin(2) region of XMMUJ0338.7+0030 imaged at all GROND bands clearly exceed those computed in deep fields/survey areas at similar to 20.5-22.5 AB mag. The photo-z's of the three imaged spectroscopic members yield z = 1.12+/-0.09 (1 sigma). The spatial distribution and the properties of the GROND sources with a photo-z in the range 1.01-1.23 confirm the correspondence of the X-ray source with a galaxy over-density at a significance of at least 4.3 sigma. Candidate members that are spectro-photometrically classified as elliptical galaxies define a red locus in the i'-z' vs. z' colour-magnitude diagram that is consistent with the red sequence of the cluster RDCS J0910+5422 at z = 1.106. XMMUJ0338.7+0030 hosts also a population of bluer late-type spirals and irregulars. The starbursts among the photometric members populate both loci, consistently with previous results. Conclusions. The analysis of the available data set indicates that XMMUJ0338.7+0030 is a low-mass cluster (M-200 similar to 10(14) M-circle dot) at z = 1.1. With the photometric accuracy yielded by the present unoptimized multi-band observations with GROND, we not only confirm the spectroscopic redshift of this cluster but also show that it hosts a galaxy population that can still undergo significant bursts of star-formation activity.