Evidence of a link between the evolution of clusters and their AGN fraction

TL;DR

This study investigates the relationship between cluster evolution and AGN populations at z ~ 1, finding that pre-virialised clusters have higher AGN fractions, supporting the idea that galaxy interactions trigger AGN activity during early cluster stages.

Contribution

It provides observational evidence linking AGN fraction to cluster evolutionary stage, highlighting differences between virialised and pre-virialised clusters at high redshift.

Findings

Pre-virialised clusters have a high AGN overdensity (~200 times).

Virialised clusters contain few or no AGN.

Radio AGN are more clustered than X-ray AGN.

Abstract

We discuss the optical properties, X-ray detections, and Active Galactic Nucleus (AGN) populations of four clusters at z ~ 1 in the Subaru-XMM Deep Field (SXDF). The velocity distribution and plausible extended X-ray detections are examined, as well as the number of X-ray point sources and radio sources associated with the clusters. We find that the two clusters that appear virialised and have an extended X-ray detection contain few, if any, AGN, whereas the two pre-virialised clusters have a large AGN population. This constitutes evidence that the AGN fraction in clusters is linked to the clusters' evolutionary stage. The number of X-ray AGN in the pre-virialised clusters is consistent with an overdensity of factor ~ 200; the radio AGN appear to be clustered with a factor of three to six higher. The median K-band luminosities of L_K = 1.7 +/- 0.7 L* for the X-ray sources and L_K = 2.3 +/- 0.1 L* for the radio sources support the theory that these AGN are triggered by galaxy interaction and merging events in sub-groups with low internal velocity distributions, which make up the cluster environment in a pre-virialisation evolutionary stage.