The AGN Population in X-ray Selected Galaxy Groups at $0.5 < z < 1.1$
  We use Chandra data to study the incidence and properties of Active Galactic
Nuclei (AGN) in 16 intermediate redshift ($0.5 < z < 1.1$) X-ray-selected
galaxy groups in the Chandra Deep Field-South. We measure an AGN fraction of
$f(L_{X,H} > 10^{42}; M_R<-20) = 8.0_{-2.3}^{+3.0}\%$ at $\bar{z} \sim 0.74$,
approximately a factor of two higher than the AGN fraction found for rich
clusters at comparable redshift. This extends the trend found at low redshift
for groups to have higher AGN fractions than clusters. Our estimate of the AGN
fraction is also more than a factor of 3 higher than that of low redshift
X-ray-selected groups. Using optical spectra from various surveys, we also
constrain the properties of emission-line selected AGN in these groups.
Contrary to the large population of X-ray AGN ($N(L_{X,H} > 10^{41}$ erg/s) =
25), we find only 4 emission-line AGN, 3 of which are also X-ray bright.
Furthermore, most of the X-ray AGN in our groups are optically-dull (i.e. lack
strong emission-lines) similar to those found in low redshift X-ray groups and
clusters of galaxies. This contrasts with the AGN population found in low
redshift optically-selected groups which are dominated by emission-line AGN.
The differences between the optically and X-ray-selected AGN populations in
groups are consistent with a scenario where most AGN in the densest
environments are currently in a low accretion state.
