  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 ~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.
