Effects of gravitational-wave recoil on the dynamics and growth of supermassive black holes

Simulations of binary black hole mergers indicate that asymmetrical gravitational wave (GW) emission can cause black holes to recoil at speeds up to thousands of km s⁻¹. These GW recoil events can dramatically affect the co-evolution of recoiling supermassive black holes (SMBHs) and their host galaxies. However, theoretical studies of SMBH–galaxy evolution almost always assume a stationary central black hole. In light of the numerical results on GW recoil velocities, we relax that assumption here and consider the consequences of recoil for SMBH evolution. We follow the trajectories of SMBHs ejected in a smooth background potential that includes both a stellar bulge and a multicomponent gaseous disc. In addition, we calculate the accretion rate on to the SMBH as a function of time, using a hybrid prescription of viscous (α-disc) and Bondi accretion. We find that recoil kicks between 100 km s⁻¹ and the escape speed cause SMBHs to wander through the galaxy and halo for ∼10⁶–10⁹ yr before settling back to the galactic centre. However, the mass accreted during this time is roughly constant at ∼10 per cent of the initial mass, independent of the recoil velocity. This indicates that while large recoils may disrupt active galactic nuclei feedback processes, recoil itself is an effective means of regulating SMBH growth. Recoiling SMBHs may be observable as spatially or kinematically offset quasars, but finding such systems could be challenging, because the largest offsets correspond to the shortest quasar lifetimes.