# High density of active galactic nuclei in the outskirts of distant galaxy clusters

## TL;DR

This study reveals a significant excess of active galactic nuclei in the outskirts of distant massive galaxy clusters, suggesting galaxy merging as a key trigger for AGN activity at these large radii.

## Contribution

It provides the first strong observational evidence of AGN triggering in the outskirts of high-redshift massive clusters with high statistical significance.

## Key findings

- Significant excess of X-ray point sources between 2 and 2.5 R500 from cluster centers
- Evidence supports galaxy merging as the main AGN triggering mechanism in outskirts
- First detection of such AGN distribution at high redshift with high confidence

## Abstract

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.