Previous observations have shown that galaxies in clusters tend to exhibit AGN activity more frequently than field galaxies, and so the cluster environment may play a role in the triggering of AGN. However, the radial distribution of AGN in clusters has been a matter of debate: specifically, there have been claims that AGN activity is more prominent in the centers of clusters, while others do not see this.
The above figure shows the excess number of X-ray point sources (relative to blank fields) as a function of cluster-centric radius in 148 X-ray detected clusters. When the central cluster galaxy is disregarded, no excess of sources is seen in the center. Most of the clusters studied by other groups were included in this sample; the authors confirm the previous results (excess of central AGN) with those specific samples, but conclude that those results were due to sample bias and/or cosmic variance.
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