The Origin of the Spatial Distribution of X-ray luminous AGN in Massive   Galaxy Clusters
  We study the spatial distribution of a 95% complete sample of 508 X-ray point sources (XPS) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations of 51 massive galaxy clusters found in the MACS survey. Covering the redshift range z=0.3-0.7, our cluster sample is statistically complete and comprises all MACS clusters with X-ray luminosities in excess of 4.5 x 10^44 erg/s (0.1-2.4 keV, h_0=0.7, LCDM). Also studied are 20 control fields that do not contain clusters. We find the XPS surface density, computed in the cluster restframe, to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The excess, believed to be caused by AGN in the cluster, is significant at the 8.0 sigma confidence level compared to the XPS density observed at the field edges. No significant central excess is found in the control fields. To investigate the physical origin of the AGN excess, we study the radial AGN density profile for a subset of 24 virialized clusters. We find a pronounced central spike (r<0.5 Mpc), followed by a depletion region at about 1.5 Mpc, and a broad secondary excess centered at approximately the virial radius of the host clusters (~2.5 Mpc). We present evidence that the central AGN excess reflects increased nuclear activity triggered by close encounters between infalling galaxies and the giant cD-type elliptical occupying the very cluster center. By contrast, the secondary excess at the cluster-field interface is likely due to black holes being fueled by galaxy mergers. In-depth spectroscopic and photometric follow-up observations of the optical counterparts of the XPS in a subset of our sample are being conducted to confirm this picture. 
Joshua T. Ruderman and Harald Ebeling