Evaluating the merger rate of binary black holes from direct captures and third-body soft interactions using the Milky Way globular clusters

Abstract

The multitude of binary black hole coalescence detections in gravitational waves has renewed our interest on environments that can be the cradle of these mergers. In this work we study merger rates of binary black holes in globular clusters that are among the most dense stellar environments and a natural place for the creation of black hole binaries. To model these systems with all their variations we rely on the observational properties of the known Milky Way globular clusters. We consider direct capture events between black holes, as well as soft interactions of black hole binaries with stars as third bodies that accelerate the evolution of these binaries. We find that binary black holes from direct captures merge at an averaged rate of 0.3 − 5 × 10 −11 yr −1 per cluster. Third body soft interactions are a much more prominent channel giving an averaged rate of 2 − 4 × 10 −10 yr −1 per cluster. Those rates in globular clusters can lead to a cumulative merger rate of about 100 mergers per year up to redshift of 1, i.e. a significant fraction of the detectable in the near future binary black hole coalescence events. Further observations of cluster properties both in terms of their masses, profile properties, velocity dispersion of stars and their cosmological distribution, will allow us to better constrain the contribution of these environments to the detectable coalescence events rate.

Black Holes merging with Low Mass Gap Objects inside Globular Clusters

“…In the Milky Way we have observed so far more than 160 such systems for many of which we have wellmeasured total mass and density profiles [1,2]. Moreover, in some of these environments the compact objects will have multiple interactions as has been shown in [3][4][5][6][7][8][9][10]. Thus, we expect different combinations of binaries between the stars and the compact objects and the compact objects themselves, which may even lead to runaway mergers [11][12][13][14][15][16][17][18][19].…”

“…In particular, if for instance e GW = 0.9, then the above timescale is about 440Myr. For a numerical description of the 3rd-body and gravitational wave emission inside realistic globular cluster environments one can see [9,10].…”

The Merger Rate of Black Holes in a Primordial Black Hole Cluster

“…In the present calculations, it is assumed that the lifetime of a binary is less than the dynamical time of the PBH cluster, the approximation similar to one used in Refs. [14,17,22].…”

“…This possibility was studied for globular star clusters with black holes in Refs. [17][18][19][20][21][22]. In addition, these binaries might significantly affect the cluster evolution, stop the core collapse, and cause gravothermal oscillations [40][41][42][43][44].…”

“…[7,[10][11][12][13] in the context of PBH clusters with narrow mass spectra (with the number of PBH N PBH 10 4 ) and in Refs. [14][15][16][17][18][19][20][21][22] in the context of a dense globular star cluster. The second possibility was considered for globular star clusters [23][24][25][26][27][28] and galactic nuclei [29][30][31][32][33] in the loss-cone treatment [34,35].…”