The AGN Population in X-ray Selected Galaxy Groups at $0.5 < z < 1.1$

TL;DR

This study investigates the prevalence and characteristics of AGN in intermediate redshift galaxy groups, revealing a higher AGN fraction than in clusters and low-redshift groups, with most being optically dull and in low accretion states.

Contribution

It provides new measurements of AGN fractions in X-ray-selected galaxy groups at $0.5<z<1.1$, highlighting differences from low-redshift groups and clusters, especially in optical emission-line properties.

Findings

AGN fraction in groups is about twice that in rich clusters at similar redshift.

Most X-ray AGN are optically dull, unlike low-redshift optically-selected groups.

The majority of AGN are in a low accretion state, with fewer emission-line AGN.

Abstract

We use Chandra data to study the incidence and properties of Active Galactic Nuclei (AGN) in 16 intermediate redshift ($0.5 < z < 1.1$) X-ray-selected galaxy groups in the Chandra Deep Field-South. We measure an AGN fraction of $f(L_{X,H} > 10^{42}; M_R<-20) = 8.0_{-2.3}^{+3.0}\%$ at $\bar{z} \sim 0.74$, approximately a factor of two higher than the AGN fraction found for rich clusters at comparable redshift. This extends the trend found at low redshift for groups to have higher AGN fractions than clusters. Our estimate of the AGN fraction is also more than a factor of 3 higher than that of low redshift X-ray-selected groups. Using optical spectra from various surveys, we also constrain the properties of emission-line selected AGN in these groups. Contrary to the large population of X-ray AGN ($N(L_{X,H} > 10^{41}$ erg/s) = 25), we find only 4 emission-line AGN, 3 of which are also X-ray bright. Furthermore, most of the X-ray AGN in our groups are optically-dull (i.e. lack strong emission-lines) similar to those found in low redshift X-ray groups and clusters of galaxies. This contrasts with the AGN population found in low redshift optically-selected groups which are dominated by emission-line AGN. The differences between the optically and X-ray-selected AGN populations in groups are consistent with a scenario where most AGN in the densest environments are currently in a low accretion state.