High density of active galactic nuclei in the outskirts of distant
  galaxy clusters
  We present a study of the distribution of X-ray detected active galactic
nuclei (AGN) in the five most massive, $M_{500}^{SZ}>10^{14} M_{\odot}$ , and
distant, z$\sim$1, galaxy clusters in the \textit{Planck} and South Pole
Telescope (SPT)\textit{} surveys. The spatial and thermodynamic individual
properties of each cluster have been defined with unprecedented accuracy at
this redshift using deep X-ray observations. This is an essential property of
our sample in order to precisely determine the $R_{500}^{Y_{\textrm x}}$ radius
of the clusters. For our purposes, we computed the X-ray point-like source
surface density in 0.5$R_{500}^{Y_{\textrm x}}$ wide annuli up to a
clustercentric distance of 4$R_{500}^{Y_{\textrm x}}$, statistically
subtracting the background and accounting for the respective average density of
optical galaxies. We found a significant excess of X-ray point sources between
2 and 2.5$R_{500}^{Y_{\textrm x}}$ at the 99.9\% confidence level. The results
clearly display for the first time strong observational evidence of AGN
triggering in the outskirts of high-redshift massive clusters with such a high
statistical significance. We argue that the particular conditions at this
distance from the cluster centre increase the galaxy merging rate, which is
probably the dominant mechanism of AGN triggering in the outskirts of massive
clusters.
