Off-Nuclear AGNs as a Signature of Recoiling Massive Black Holes

Abstract

During the final phases of inspiral, a massive black hole (MBH) binary experiences a recoil due to the asymmetric emission of gravitational waves. We use recent results from numerical relativity simulations together with models of the assembly and growth of MBHs in hierarchical cosmologies, to study the dynamics, statistics, and observability of recoiling MBHs. We find that, at redshift z<3, kicked nonrotating holes are typically found between 1 and 30 kpc from their galaxy centers, while rapidly rotating ones are typically between 10 and a few hundred kpc. A recoiling hole that carries an accretion disk may shine as an ``off-nuclear AGN'' while it moves away from the center of its host galaxy. We predict that, depending on the hole spin distribution and the duration of their active phase, a population of off-nuclear AGNs may already be detectable at low and intermediate redshifts in present deep Hubble Space Telescope observations. The James Webb Space Telescope may discover tens of wandering AGNs per square degree, most of them moving within their host halos on unbound trajectories.