The origin of the spatial distribution of X-ray-luminous active galactic nuclei in massive galaxy clusters

We study the spatial distribution of a 95% complete sample of 508 X-ray point sources (XPSs) detected in the 0.5-2.0 keV band in Chandra ACIS-I observations of 51 massive galaxy clusters found in the MAssive Cluster Survey (MACS). 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 × 10⁴⁴ ergs s^-1 (0.1-2.4 keV, h₀ = 0.7, λCDM). Also studied are 20 control fields that do not contain clusters. We find the XPS surface density, computed in the cluster rest frame, to exhibit a pronounced excess within 3.5 Mpc of the cluster centers. The excess, believed to be caused by active galactic nuclei (AGNs) in the cluster, is significant at the 8.0 σ 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 galaxy 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 XPSs in a subset of our sample are being conducted to confirm this picture.