Large Merger Recoils and Spin Flips From Generic Black-Hole Binaries

TL;DR

This study presents the first simulations of generic black-hole binaries with unequal masses and misaligned spins, revealing large recoil velocities and spin flips that could eject merged black holes from galaxies.

Contribution

It provides novel numerical results on recoil velocities and spin dynamics in generic black-hole mergers with misaligned spins.

Findings

Remnant receives a recoil of 454 km/s in one configuration.

A different configuration yields a recoil of 1830 km/s, potentially scaling to 4000 km/s.

Significant spin flips of the remnant black hole observed.

Abstract

We report the first results from evolutions of a generic black-hole binary, i.e. a binary containing unequal mass black holes with misaligned spins. Our configuration, which has a mass ratio of 2:1, consists of an initially non-spinning hole orbiting a larger, rapidly spinning hole (specific spin a/m = 0.885), with the spin direction oriented -45 degrees with respect to the orbital plane. We track the inspiral and merger for ~2 orbits and find that the remnant receives a substantial kick of 454 km/s, more than twice as large as the maximum kick from non-spinning binaries. The remnant spin direction is flipped by 103 degrees with respect to the initial spin direction of the larger hole. We performed a second run with anti-aligned spins, a/m = +-0.5 lying in the orbital plane that produces a kick of 1830 km/s off the orbital plane. This value scales to nearly 4000 km/s for maximally spinning holes. Such a large recoil velocity opens the possibility that a merged binary can be ejected even from the nucleus of a massive host galaxy.