  Hubble Space Telescope images of the galaxy cluster Abell 2261, obtained as
part of the Cluster Lensing And Supernova survey with Hubble, show that the
brightest galaxy in the cluster, A2261-BCG, has the largest core yet detected
in any galaxy. The cusp radius of A2261-BCG is 3.2 kpc, twice as big as the
next largest core known, and ~3x bigger than those typically seen in the most
luminous BCGs. The morphology of the core in A2261-BCG is also unusual, having
a flat or even slightly-depressed interior surface brightness profile, rather
than the typical shallow cusp. This implies that the galaxy has a core with
constant or even centrally decreasing stellar density. Interpretation of the
core as an end product of the "scouring" action of a binary supermassive black
hole implies a total black hole mass ~1E+10 M_sun from the extrapolation of
most relationships between core structure and black hole mass. The core falls
1-sigma above the cusp-radius versus galaxy luminosity relation. Its large size
in real terms, and the extremely large black hole mass required to generate it,
raise the possibility that the core has been enlarged by additional processes,
such as the ejection of the black holes that originally generated the core. The
flat central stellar density profile is consistent with this hypothesis. The
core is also displaced by 0.7 kpc from the center of the surrounding envelope,
consistent with a local dynamical perturbation of the core.
