A Displaced Supermassive Black Hole in M87
  Isophotal analysis of M87, using data from the Advanced Camera for Surveys,
reveals a projected displacement of 6.8 +/- 0.8 pc (~ 0.1 arcsec) between the
nuclear point source (presumed to be the location of the supermassive black
hole, SMBH) and the photo-center of the galaxy. The displacement is along a
position angle of 307 +/- 17 degrees and is consistent with the jet axis. This
suggests the active SMBH in M87 does not currently reside at the galaxy center
of mass, but is displaced in the counter-jet direction. Possible explanations
for the displacement include orbital motion of an SMBH binary, gravitational
perturbations due to massive objects (e.g., globular clusters), acceleration by
an asymmetric or intrinsically one-sided jet, and gravitational recoil
resulting from the coalescence of an SMBH binary. The displacement direction
favors the latter two mechanisms. However, jet asymmetry is only viable, at the
observed accretion rate, for a jet age of >0.1 Gyr and if the galaxy restoring
force is negligible. This could be the case in the low density core of M87. A
moderate recoil ~1 Myr ago might explain the disturbed nature of the nuclear
gas disk, could be aligned with the jet axis, and can produce the observed
offset. Alternatively, the displacement could be due to residual oscillations
resulting from a large recoil that occurred in the aftermath of a major merger
any time in the last 1 Gyr.
