A Radio Relic and a Search for the Central Black Hole in the Abell 2261
  Brightest Cluster Galaxy
  We present VLA images and HST/STIS spectra of sources within the center of
the brightest cluster galaxy (BCG) in Abell 2261. These observations were
obtained to test the hypothesis that its extremely large, flat core reflects
the ejection of its supermassive black hole. Spectra of three of the four most
luminous "knots" embedded in the core were taken to test whether one may
represent stars bound to a displaced massive black hole. The three knots have
radial velocity offsets dV < ~150 km/s from the BCG. Knots 2 and 3 show
kinematics, colors, and stellar masses consistent with infalling low-mass
galaxies or larger stripped cluster members. Large errors in the stellar
velocity dispersion of Knot 1, however, mean that we cannot rule out the
hypothesis that it hosts a high-mass black hole. A2261-BCG has a compact, relic
radio-source offset by 6.5 kpc (projected) from the optical core's center, but
no active radio core that would pinpoint the galaxy's central black hole to a
tight 10 GHz flux limit <3.6 uJy. Its spectrum and morphology are suggestive of
an active galactic nucleus that switched off >48 Myr ago, with an equipartition
condition magnetic field of 15 uG. These observations are still consistent with
the hypothesis that the nuclear black hole has been ejected from its core, but
the critical task of locating the supermassive black hole or demonstrating that
A2261-BCG lacks one remains to be done.
