The Cosmic Evolution of AGN in galaxy clusters

TL;DR

This study investigates the evolution of active galactic nuclei (AGN) in galaxy clusters over redshift, revealing an increasing AGN density in clusters at higher redshifts, especially for X-ray selected AGN, indicating a Butcher-Oemler type effect.

Contribution

It provides the first comprehensive analysis of AGN surface density in galaxy clusters across a wide redshift range using multiple selection methods.

Findings

AGN overdensity in clusters increases with redshift

X-ray selected AGN are three times more common in high-redshift clusters

The AGN increase exceeds expectations from cluster richness evolution

Abstract

We present the surface density of luminous active galactic nuclei (AGN) associated with a uniformly selected galaxy cluster sample identified in the 8.5 square degree Bootes field of the NOAO Deep Wide-Field Survey. The clusters are distributed over a large range of redshift (0 < z < 1.5) and we identify AGN using three different selection criteria: mid-IR color, radio luminosity, and X-ray luminosity. Relative to the field, we note a clear overdensity of the number of AGN within 0.5 Mpc of the cluster centers at z > 0.5. The amplitude of this AGN overdensity increases with redshift. Although there are significant differences between the AGN populations probed by each selection technique, the rise in cluster AGN surface density generally increases more steeply than that of field quasars. In particular, X-ray selected AGN are at least three times more prevalent in clusters at 1 < z < 1.5 compared to clusters at 0.5 < z < 1. This effect is stronger than can be explained by the evolving median richness of our cluster sample. We thus confirm the existence of a Butcher-Oemler type effect for AGN in galaxy clusters, with the number of AGN in clusters increasing with redshift.