Relativistic suppression of black hole recoils

Abstract

Numerical-relativity simulations indicate that the black hole produced in a binary merger can recoil with a velocity up to ν max ≃ 4000kms-1 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 (SBHs) 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 paper, 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° while the second spin S 2 is isotropically distributed, the spin alignment discussed in this paper reduces the median recoil from 864kms-1 to 273kms-1 for maximally spinning black holes with a mass ratio of 9/11. This reduction will greatly increase the fraction of galaxies retaining their SBHs. © 2010. The American Astronomical Society. All rights reserved.