Unexpectedly high fraction of active galactic nuclei in red cluster galaxies

As part of a program to study the evolution of active galactic nuclei (AGNs) in clusters of galaxies, we present our results for A2104. A deep Chandra observation of this massive, z=0.154 cluster reveals a significant X-ray point-source excess over the expectations of blank fields, including eight X-ray counterparts with R<20 mag. Our spectroscopy shows that all six X-ray sources associated with red counterparts are cluster members and their X-ray properties are consistent with all of them being AGNs. Only one of the six has the emission lines characteristic of optically selected AGNs; the remaining five would not have been classified as AGNs based on their optical spectra. This suggests the existence of a large population of obscured, or at least optically unremarkable, AGNs in clusters of galaxies. These six sources correspond to a lower limit of ∼5% of the AGN fraction in cluster galaxies with R<20 mag (rest-frame M-v = -19.5 mag) and are comparable to the blue galaxy fraction in the cluster. Such an obscured AGN population in clusters of galaxies has many implications for cluster galaxy evolution, the hidden growth of their central, supermassive black holes, estimates of the star formation rate at infrared and radio wavelengths, and the observed variance in the hard X-ray background.