Hiding in Plain Sight - Recovering Clusters of Galaxies with the
  Strongest AGN in Their Cores
  A key challenge in understanding the feedback mechanism of AGN in Brightest
Cluster Galaxies (BCGs) is the inherent rarity of catching an AGN during its
strong outburst phase. This is exacerbated by the ambiguity of differentiating
between AGN and clusters in X-ray observations. If there is evidence for an AGN
then the X-ray emission is commonly assumed to be dominated by the AGN
emission, introducing a selection effect against the detection of AGN in BCGs.
In order to recover these 'missing' clusters, we systematically investigate the
colour-magnitude relation around some ~3500 ROSAT All Sky Survey selected AGN,
looking for signs of a cluster red sequence. Amongst our 22 candidate systems,
we independently rediscover several confirmed systems, where a strong AGN
resides in a central galaxy. We compare the X-ray luminosity to red sequence
richness distribution of our AGN candidate systems with that of a similarly
selected comparison sample of ~1000 confirmed clusters and identify seven
'best' candidates (all of which are BL Lac objects), where the X-ray flux is
likely to be a comparable mix between cluster and AGN emission. We confirm that
the colours of the red sequence are consistent with the redshift of the AGN,
that the colours of the AGN host galaxy are consistent with AGN, and, by
comparing their luminosities with those from our comparison clusters, confirm
that the AGN hosts are consistent with BCGs.
