As part of a program to study the evolution of active galactic nuclei ( AGN ) in clusters of galaxies , we present our results for Abell 2104 .
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 AGN .
Only one of the six has the emission lines characteristic of optically selected AGN ; the remaining five would not have been classified as AGN based on their optical spectra .
This suggests the existence of a large population of obscured , or at least optically unremarkable , AGN in clusters of galaxies .
These six sources correspond to a lower limit of \sim 5 % of the AGN fraction in cluster galaxies with R < 20 mag ( rest-frame M _ { V } = -19.5 mag ) and is 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 .