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 XMMU J0338.7 + 0030, suggested to be at z ~ 1.45 ± 0.15 (1σ) 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 (1σ). To reach a better knowledge of its galaxy population, we observed XMMU J0338.7 + 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' ~ 26 AB mag (1σ) in the 3.9 × 4.3 arcmin² region of XMMU J0338.7 + 0030 imaged at all GROND bands clearly exceed those computed in deep fields/survey areas at ~20.5-22.5 AB mag. The photo-z's of the three imaged spectroscopic members yield z = 1.12 ± 0.09 (1σ). 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 4.3σ. 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. XMMU J0338.7 + 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 XMMU J0338.7 + 0030 is a low-mass cluster (M₂₀₀ ~ 10¹⁴ M_⊙) 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.

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