Relativistic Suppression of Black Hole Recoils
  Numerical-relativity simulations indicate that the black hole produced in a
binary merger can recoil with a velocity up to v_max ~ 4,000 km/s with respect
to the center of mass of the initial binary. This challenges the paradigm that
most galaxies form through hierarchical mergers, yet retain supermassive black
holes at their centers despite having escape velocities much less than v_max.
Interaction with a circumbinary disk can align the binary black hole spins with
their orbital angular momentum, reducing the recoil velocity of the final black
hole produced in the subsequent merger. However, the effectiveness of this
alignment depends on highly uncertain accretion flows near the binary black
holes. In this Letter, we show that if the spin S_1 of the more massive binary
black hole is even partially aligned with the orbital angular momentum L,
relativistic spin precession on sub-parsec scales can align the binary black
hole spins with each other. This alignment significantly reduces the recoil
velocity even in the absence of gas. For example, if the angle between S_1 and
L at large separations is 10 degrees while the second spin S_2 is isotropically
distributed, the spin alignment discussed in this paper reduces the median
recoil from 864 km/s to 273 km/s for maximally spinning black holes with a mass
ratio of 9/11. This reduction will greatly increase the fraction of galaxies
retaining their supermassive black holes.
