Off-nuclear AGN as a signature of recoiling massive black holes

TL;DR

This paper investigates the dynamics and observability of recoiling massive black holes, which can appear as off-nuclear active galactic nuclei (AGN), using simulations and models to predict their distribution and detectability.

Contribution

It combines numerical relativity simulations with cosmological models to analyze the distribution, dynamics, and potential observability of recoiling MBHs as off-nuclear AGN.

Findings

Recoiling MBHs are typically found 1-30 kpc from galaxy centers at z<3.

Rapidly rotating MBHs can be located hundreds of kpc from centers.

A population of off-nuclear AGN may already be detectable with current telescopes.

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 recoling MBHs. We find that, at redshift z<3, kicked non-rotating 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 AGN 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 AGN per square degree, most of them moving within their host halos on unbound trajectories.