The Clusters Hiding in Plain Sight (CHiPS) survey: Complete sample of
  extreme BCG clusters
  We present optical follow-up observations for candidate clusters in the
Clusters Hiding in Plain Sight (CHiPS) survey, which is designed to find new
galaxy clusters with extreme central galaxies that were misidentified as bright
isolated sources in the ROSAT All-Sky Survey catalog. We identify 11 cluster
candidates around X-ray, radio, and mid-IR bright sources, including six
well-known clusters, two false associations of foreground and background
clusters, and three new candidates which are observed further with Chandra. Of
the three new candidates, we confirm two newly discovered galaxy clusters:
CHIPS1356-3421 and CHIPS1911+4455. Both clusters are luminous enough to be
detected in the ROSAT All Sky-Survey data if not because of their bright
central cores. CHIPS1911+4455 is similar in many ways to the Phoenix cluster,
but with a highly-disturbed X-ray morphology on large scales. We find the
occurrence rate for clusters that would appear to be X-ray bright point sources
in the ROSAT All-Sky Survey (and any surveys with similar angular resolution)
to be 2+/-1%, and the occurrence rate of clusters with runaway cooling in their
cores to be <1%, consistent with predictions of Chaotic Cold Accretion. With
the number of new groups and clusters predicted to be found with eROSITA, the
population of clusters that appear to be point sources (due to a central QSO or
a dense cool core) could be around 2000. Finally, this survey demonstrates that
the Phoenix cluster is likely the strongest cool core at z<0.7 -- anything more
extreme would have been found in this survey.
