Lower AGN Abundance in Galaxy Clusters at z < 0.5
  Most galaxies in clusters have supermassive black holes at their center, and
a fraction of those supermassive black holes show strong activity. These active
galactic nuclei(AGNs) are an important probe of environmental dependence of
galaxy evolution, intra-cluster medium, and cluster-scale feedback. We
investigated AGN fraction in one of the largest samples of X-ray selected
clusters from the ROSAT and their immediate surrounding field regions below z <
0.5. We found lower average AGN fraction in clusters, (2.37+-0.39)% than for
the fields (5.12+-0.16)%. The lower AGN fractions in clusters were measured,
after dividing the clusters into five redshift intervals between 0.0 and 0.5,
in each redshift interval, and we found an increase in the fraction for both
cluster and field galaxies with redshift below z < 0.5, which clearly indicates
an environment and redshift dependence. We further divided the clusters into
low-mass and high-mass objects using a mass cut at log(M500/Msun) = 13.5,
finding comparable AGN fractions for both classifications, while a
significantly higher AGN fraction in field. We also measured increasing AGN
fractions with clustercentric distance for all redshift bins, further
confirming the environmental dependence of AGN activities. In addition, we did
not find an obvious trend between AGN fraction and SDSS-R absolute magnitudes
among different redshift bins. We conclude that the lower AGN fraction in
clusters relative to fields indicate that factors, such as inefficient galaxy
mergers and ram pressure stripping cause a deficit of cold gas available in
high density regions to fuel the central super-massive black hole. Clusters and
fields in present universe have lost more gas relative to their high redshift
counterparts resulting in a lower AGN fraction observed today.
