Star clusters around recoiled black holes in the Milky Way halo

Ryan M O'Leary,Abraham Loeb

doi:10.1111/j.1365-2966.2009.14611.x

Ryan M O'Leary, Abraham Loeb

Open Access

https://doi.org/10.1111/j.1365-2966.2009.14611.x

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Abstract

Gravitational wave emission by coalescing black holes (BHs) kicks the remnant BH with a typical velocity of hundreds of km s -1 . This velocity is sufficiently large to remove the remnant BH from a low-mass galaxy but is below the escape velocity from the Milky Way (MW) galaxy. If central BHs were common in the galactic building blocks that merged to make the MW, then numerous BHs that were kicked out of low-mass galaxies should be freely floating in the MW halo today. We use a large statistical sample of possible merger tree histories for the MW to estimate the expected number of recoiled BH remnants present in the MW halo today. We find that hundreds of BHs should remain bound to the MW halo after leaving their parent low-mass galaxies. Each BH carries a compact cluster of old stars that populated the core of its original host galaxy. Using the time-dependent Fokker-Planck equation, we find that the present-day clusters are ≤ 1 pc in size, and their central bright regions should be unresolved in most existing sky surveys. These compact systems are distinguishable from globular clusters by their internal (Keplerian) velocity dispersion greater than 100 km s -1 and their high mass-to-light ratio owing to the central BH. An observational discovery of this relic population of star clusters in the MW halo would constrain the formation history of the MW and the dynamics of BH mergers in the early Universe. A similar population should exist around other galaxies and may potentially be detectable in M31 and M33.

Highlights

During the final coalescence of two black holes (BHs), gravitational waves (GWs) are emitted unisotropically and carry away linear momentum, kicking the merger BH remnant in the opposite direction (Peres 1962; Bekenstein 1973; Fitchett 1983)
The compact star clusters around ejected BHs are distinguishable from cores of globular clusters by their internal (Keplerian) velocity dispersion of order 100 km s−1 and their high mass-to-light ratio owing to the central BH
Based on a large statistical ensemble of merger tree histories for the Milky Way (MW), we have found that hundreds of GW recoiled BHs should reside within the MW halo today

Summary

INTRODUCTION

During the final coalescence of two black holes (BHs), gravitational waves (GWs) are emitted unisotropically and carry away linear momentum, kicking the merger BH remnant in the opposite direction (Peres 1962; Bekenstein 1973; Fitchett 1983). The resulting kick velocity of typically hundreds of km s−1 depends on the mass ratio of the BHs as well as the spin and orientation of the binary before coalescence (Baker et al 2006; Campanelli et al 2007a,b; Tichy & Marronetti 2007) Such kicks can alter both the population of nuclear BHs in galactic bulges (Madau & Quataert 2004; Libeskind et al 2006; Schnittman 2007; Volonteri 2007; Blecha & Loeb 2008) as well as the core of stars in the bulge itself (Gualandris & Merritt 2008).

THE MERGER HISTORY OF THE MW AND EJECTED BHS IN THE HALO

STRUCTURE AND EVOLUTION OF EJECTED STAR CLUSTERS

OBSERVING EJECTED SYSTEMS

SUMMARY AND DISCUSSION