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

TL;DR

This study investigates how gravitational-wave recoil impacts supermassive black hole trajectories and growth, revealing that recoil can cause SMBHs to wander significantly and regulate their mass accumulation, affecting galaxy evolution.

Contribution

It introduces a dynamic model of recoiling SMBHs considering realistic galaxy potentials and accretion, highlighting the effects of GW recoil on SMBH evolution and observability.

Findings

Recoil velocities between 100 km/s and escape speed cause SMBHs to wander for up to 1 Gyr.

Mass accreted during recoil is about 10% of initial SMBH mass, regardless of recoil velocity.

Recoil can disrupt active galactic nuclei feedback but also regulate SMBH growth.

Abstract

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. These GW recoil events can dramatically affect the coevolution 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 multi-component gaseous disk. In addition, we calculate the accretion rate onto the SMBH as a function of time using a hybrid prescription of viscous (alpha-disk) and Bondi accretion. We find that recoil kicks between 100 km/s and the escape speed cause SMBHs to wander through the galaxy and halo for about 1 Myr - 1 Gyr before settling back to the galactic center. However, the mass accreted during this time is roughly constant at about 10% 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.