Gravitational Wave Recoil and the Retention of Intermediate Mass Black Holes

TL;DR

This paper studies how gravitational wave recoil affects the retention of intermediate mass black holes in globular clusters, revealing that most IMBHs are likely ejected, which could explain rogue black holes and ULXs.

Contribution

It provides the first detailed analysis of IMBH retention probabilities considering gravitational recoil effects in globular clusters.

Findings

Less than 3% of clusters retain IMBHs >1000 solar masses with uniform spin assumptions.

Up to 16% retention when accounting for stellar wind and supernova mass loss.

Repeated IMBH-BH encounters can eject IMBHs with over 30% probability.

Abstract

During the inspiral and merger of a binary black hole, gravitational radiation is emitted anisotropically due to asymmetries in the merger configuration. This anisotropic radiation leads to a gravitational wave kick, or recoil velocity, as large as ~ 4000 km/sec. We investigate the effect gravitational recoil has on the retention of intermediate mass black holes (IMBH) within Galactic globular clusters. Assuming that our current understanding of IMBH-formation is correct and yields an IMBH-seed in every globular cluster, we find a significant problem retaining low mass IMBHs (1000 $\Msun$) in the typical merger-rich globular cluster environment. Given a uniform black hole spin distribution and orientation and a Kroupa IMF, we find that at most 3% of the globular clusters can retain an IMBH larger than 1000 $\Msun$ today. For a population of black holes that better approximates mass loss from winds and supernovae, we find that 16% of globulars can retain an IMBH larger than 1000 $\Msun$. Our calculations show that if there are black holes of mass $M > 60 \Msun$ in a cluster, repeated IMBH-BH encounters will eventually eject a 1000 $\Msun$ IMBH with greater than 30% probability. As a consequence, a large population of rogue black holes may exist in our Milky Way halo. We discuss the dynamical implications of this subpopulation, and its possible connection to ultraluminous X-ray sources (ULXs).