Active Galactic Nuclei in Groups and Clusters of Galaxies: Detection and
  Host Morphology
  The incidence and properties of Active Galactic Nuclei (AGN) in the field,
groups, and clusters can provide new information about how these objects are
triggered and fueled, similar to how these environments have been employed to
study galaxy evolution. We have obtained new XMM-Newton observations of seven
X-ray selected groups and poor clusters with 0.02 < z < 0.06 for comparison
with previous samples that mostly included rich clusters and optically-selected
groups. Our final sample has ten groups and six clusters in this low-redshift
range (split at a velocity dispersion of $\sigma = 500$ km/s). We find that the
X-ray selected AGN fraction increases from $f_A(L_X>10^{41}; M_R<M_R^*+1) =
0.047^{+0.023}_{-0.016}$ in clusters to $0.091^{+0.049}_{-0.034}$ for the
groups (85% significance), or a factor of two, for AGN above an 0.3-8keV X-ray
luminosity of $10^{41}$ erg/s hosted by galaxies more luminous than $M_R^*+1$.
The trend is similar, although less significant, for a lower-luminosity host
threshold of $M_R = -20$ mag. For many of the groups in the sample we have also
identified AGN via standard emission-line diagnostics and find that these AGN
are nearly disjoint from the X-ray selected AGN. Because there are substantial
differences in the morphological mix of galaxies between groups and clusters,
we have also measured the AGN fraction for early-type galaxies alone to
determine if the differences are directly due to environment, or indirectly due
to the change in the morphological mix. We find that the AGN fraction in
early-type galaxies is also lower in clusters $f_{A,n>2.5}(L_X>10^{41};
M_R<M_R^*+1) = 0.048^{+0.028}_{-0.019}$ compared to $0.119^{+0.064}_{-0.044}$
for the groups (92% significance), a result consistent with the hypothesis that
the change in AGN fraction is directly connected to environment.
