First Measurement of a Rapid Increase in the AGN Fraction in High-Redshift Clusters of Galaxies

TL;DR

This study measures the increase in active galactic nuclei (AGN) within high-redshift galaxy clusters, revealing a significant rise compared to lower-redshift clusters, indicating evolving galaxy activity in dense environments.

Contribution

First measurement of AGN fraction in high-redshift clusters (z~0.6) showing a substantial increase over lower-redshift clusters, highlighting evolution in galaxy activity within clusters.

Findings

AGN fraction at z~0.6 is approximately 20 times higher than in low-redshift clusters.

Significant increase in AGN activity suggests an AGN Butcher-Oemler Effect.

Cluster AGN activity evolves more rapidly than in the field.

Abstract

We present the first measurement of the AGN fraction in high-redshift clusters of galaxies (z~0.6) with spectroscopy of one cluster and archival data for three additional clusters. We identify 8 AGN in all four of these clusters from the Chandra data, which are sensitive to AGN with hard X-ray (2-10keV) luminosity L_{X,H} > 10^43 erg/s in host galaxies more luminous than a rest frame M_R < -20 mag. This stands in sharp contrast to the one AGN with L_{X,H} > 10^43 erg/s we discovered in our earlier study of eight low-redshift clusters with z=0.06-0.31 (average z~0.2). Three of the four high-redshift cluster datasets are sensitive to nearly L_{X,H} > 10^42 erg/s and we identify seven AGN above this luminosity limit, compared to two in eight, low-redshift clusters. Based on membership estimates for each cluster, we determine that the AGN fraction at z~0.6 is f_A(L_X>10^42;M_R<-20) = 0.028 (+0.019/-0.012) and f_A(L_X>10^43;M_R<-20) = 0.020 (+0.012/-0.008). These values are approximately a factor of 20 greater than the AGN fractions in lower-redshift (average z~0.2) clusters of galaxies and represent a substantial increase over the factors of 1.5 and 3.3 increase, respectively, in the measured space density evolution of the hard X-ray luminosity function over this redshift range. Potential systematic errors would only increase the significance of our result. The cluster AGN fraction increases more rapidly with redshift than the field and the increase in cluster AGN indicates the presence of an AGN Butcher-Oemler Effect.