The Clusters Hiding in Plain Sight (CHiPS) survey: CHIPS1911+4455, a
  Rapidly-Cooling Core in a Merging Cluster
  We present high-resolution optical images from the Hubble Space Telescope,
X-ray images from the Chandra X-ray Observatory, and optical spectra from the
Nordic Optical Telescope for a newly-discovered galaxy cluster, CHIPS1911+4455,
at z=0.485+/-0.005. CHIPS1911+4455 was discovered in the Clusters Hiding in
Plain Sight (CHiPS) survey, which sought to discover galaxy clusters with
extreme central galaxies that were misidentified as isolated X-ray point
sources in the ROSAT All-Sky Survey. With new Chandra X-ray observations, we
find the core (r=10 kpc) entropy to be 17+2-9 keV cm^2, suggesting a strong
cool core, which are typically found at the centers of relaxed clusters.
However, the large-scale morphology of CHIPS1911+4455 is highly asymmetric,
pointing to a more dynamically active and turbulent cluster. Furthermore, the
Hubble images reveal a massive, filamentary starburst near the brightest
cluster galaxy (BCG). We measure the star formation rate for the BCG to be
140--190 Msun/yr, which is one of the highest rates measured in a central
cluster galaxy to date. One possible scenario for CHIPS1911+4455 is that the
cool core was displaced during a major merger and rapidly cooled, with cool,
star-forming gas raining back toward the core. This unique system is an
excellent case study for high-redshift clusters, where such phenomena are
proving to be more common. Further studies of such systems will drastically
improve our understanding of the relation between cluster mergers and cooling,
and how these fit in the bigger picture of active galactic nuclei (AGN)
feedback.
