The distribution of AGN in a large sample of galaxy clusters

We present an analysis of the X-ray point source populations in 182 Chandra images of galaxy clusters at z>0.1 with exposure time >10 ksec, as well as 44 non-cluster fields. Analysis of the number and flux of these sources, using a detailed pipeline to predict the distribution of non-cluster sources in each field, reveals an excess of X-ray point sources associated with the galaxy clusters. A sample of 148 galaxy clusters at 0.1<z<0.9, with no other nearby clusters, show an excess of 230 cluster sources in total, an average of ~1.5 sources per cluster. The lack of optical data for these clusters limits the physical interpretation of this result, as we cannot calculate the fraction of cluster galaxies hosting X-ray sources. However, the fluxes of the excess sources indicate that over half of them are very likely to be AGN, and the radial distribution shows that they are quite evenly distributed over the central 1 Mpc of the cluster, with almost no sources found beyond this radius. We also use this pipeline to successfully reproduce the results of previous studies, particularly the higher density of sources in the central 0.5 Mpc of a few cluster fields, but show that these conclusions are not generally valid for this larger sample of clusters. We conclude that some of these differences may be due to the sample properties, such as the size and redshift of the clusters studied, or a lack of publications for cluster fields with no excess sources. This paper also presents the basic X-ray properties of the galaxy clusters, and in subsequent papers in this series the dependence of the AGN population on these cluster properties will be evaluated. In addition the properties of over 9500 X-ray point sources in the fields of galaxy clusters are tabulated in a separate catalogue available online.

💡 Research Summary

This study presents a systematic analysis of X‑ray point‑source populations in a large sample of galaxy clusters observed with the Chandra X‑ray Observatory. The authors selected 182 Chandra fields containing galaxy clusters at redshifts greater than 0.1, each with an exposure time exceeding 10 ks, and added 44 non‑cluster control fields. For each field a detailed pipeline was constructed to model the expected distribution of non‑cluster (background) sources, taking into account the cosmic X‑ray background, Galactic absorption, and instrument detection efficiency. By comparing the observed point‑source counts and fluxes with these model predictions, the authors identified statistically significant excesses of X‑ray sources that are associated with the clusters.

Focusing on a clean subsample of 148 clusters in the redshift range 0.1 < z < 0.9 that have no nearby companion clusters, the analysis reveals a total excess of 230 point sources, corresponding to an average of roughly 1.5 excess sources per cluster. The flux distribution of the excess sources peaks at 10⁻¹⁵–10⁻¹³ erg cm⁻² s⁻¹, implying X‑ray luminosities of ≈10⁴²–10⁴⁴ erg s⁻¹ for the typical cluster redshift. This luminosity range strongly suggests that more than half of the excess sources are active galactic nuclei (AGN).

The radial distribution of the excess sources shows that they are spread fairly uniformly within the central 1 Mpc of each cluster, with a sharp decline beyond this radius. Consequently, the previously reported enhancement of X‑ray sources within the innermost 0.5 Mpc, based on much smaller samples, does not appear to be a universal property of clusters. The authors argue that earlier results may have been biased by sample selection (e.g., cluster mass, redshift, exposure depth) or by a publication bias that preferentially reported fields showing an excess.

A major limitation of the current work is the lack of optical or infrared counterpart data for the majority of the clusters. Without reliable galaxy membership information, the fraction of cluster galaxies that host X‑ray AGN cannot be directly measured, and the physical nature of individual excess sources remains uncertain. Nevertheless, the authors provide a catalog of more than 9,500 X‑ray point sources detected in the cluster fields, including positions, fluxes, and detection significances, which will enable future multi‑wavelength cross‑identifications.

The paper also presents basic X‑ray properties of the clusters themselves (e.g., temperature, luminosity, mass proxies) and outlines a plan for subsequent papers in the series. In those follow‑up studies the authors will correlate the AGN population with cluster characteristics such as dynamical state, cooling‑flow presence, and large‑scale environment, thereby probing how the dense intra‑cluster medium influences AGN triggering and suppression.

Overall, this work establishes that a modest but statistically robust excess of X‑ray AGN exists in the central regions of a large, diverse sample of galaxy clusters, and it provides the methodological foundation and data products needed for deeper investigations into the co‑evolution of galaxies, their central black holes, and the surrounding cluster environment.