Observational constraints on the average radial distribution profile of AGN in distant galaxy clusters can provide important clues on the triggering mechanisms of AGN activity in dense environments and are essential for a completeness evaluation of c
luster selection techniques in the X-ray and mm-wavebands. The aim of this work is a statistical study with XMM-Newton of the presence and distribution of X-ray AGN in the large-scale structure environments of 22 X-ray luminous galaxy clusters in the redshift range 0.9 < z lesssim 1.6 compiled by the XMM-Newton Distant Cluster Project (XDCP). To this end, the X-ray point source lists from detections in the soft-band (0.35-2.4 keV) and full-band (0.3-7.5 keV) were stacked in cluster-centric coordinates and compared to average background number counts extracted from three independent control fields in the same observations. A significant full-band (soft-band) excess of sim78 (67) X-ray point sources is found in the cluster fields within an angular distance of 8 (4Mpc) at a statistical confidence level of 4.0 sigma (4.2 sigma), corresponding to an average number of detected excess AGN per cluster environment of 3.5pm0.9 (3.0pm0.7). The data point towards a rising radial profile in the cluster region (r<1Mpc) of predominantly low-luminosity AGN with an average detected excess of about one point source per system, with a tentative preferred occurrence along the main cluster elongation axis. A second statistically significant overdensity of brighter soft-band detected AGN is found at cluster-centric distances of 4-6 (2-3Mpc), corresponding to about three times the average cluster radius R200 of the systems. If confirmed, these results would support the idea of two different physical triggering mechanisms of X-ray AGN activity in dependence of the radially changing large-scale structure environment of the distant clusters.
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. ... 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 2m-class telescope. ... The test case is XMMU J0338.7+0030, suggested to be at z~1.45+/-0.15 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. To reach a better knowledge of its galaxy population, we observed XMMU J0338.7+0030 with GROND for about 6 hr. The publicly available photo-z code le Phare was used. The Ks-band number counts of the non-stellar sources out of the 832 detected down to z~26 AB-mag in the 3.9x4.3 square arcmin 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-zs of the three imaged spectroscopic members yield z=1.12+/-0.09. 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. 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. The analysis of the available data set indicates that XMMU J0338.7+0030 is a low-mass cluster (M_200 ~ 1E14 M_sun) at z=1.1. (Abridged)
