Recoiling Supermassive Black Holes: a search in the Nearby Universe
  The coalescence of a binary black hole can be accompanied by a large
gravitational recoil due to anisotropic emission of gravitational waves. A
recoiling supermassive black hole (SBH) can subsequently undergo long-lived
oscillations in the potential well of its host galaxy, suggesting that offset
SBHs may be common in the cores of massive ellipticals. We have analyzed HST
archival images of 14 nearby core ellipticals, finding evidence for small
($\lesssim 10$ pc) displacements between the AGN (locating the SBH) and the
center of the galaxy (the mean photocenter) in 10 of them. Excluding objects
that may be affected by large-scale isophotal asymmetries, we consider six
galaxies to have detected displacements, including M87, where a displacement
was previously reported by Batcheldor et al. 2010. In individual objects, these
displacements can be attributed to residual gravitational recoil oscillations
following a major or minor merger within the last few Gyr. For plausible merger
rates, however, there is a high probability of larger displacements than those
observed, if SBH coalescence took place in these galaxies. Remarkably, the
AGN-photocenter displacements are approximately aligned with the radio source
axis in four of the six galaxies with displacements, including three of the
four having relatively powerful kpc-scale jets. This suggests intrinsic
asymmetries in radio jet power as a possible displacement mechanism, although
approximate alignments are also expected for gravitational recoil. Orbital
motion in SBH binaries and interactions with massive perturbers can produce the
observed displacement amplitudes but do not offer a ready explanation for the
alignments.
