Black hole binaries dynamically formed in globular clusters

Abstract

We investigate properties of black hole (BH) binaries formed in globular clusters via dynamical processes, using direct N-body simulations. We pay attention to effects of BH mass function on the total mass and mass ratio distributions of BH binaries ejected from clusters. Firstly, we consider BH populations with two different masses in order to learn basic differences from models with single-mass BHs only. Secondly, we consider continuous BH mass functions adapted from recent studies on massive star evolution in a low metallicity environment, where globular clusters are formed. In this work, we consider only binaries that are formed by three-body processes and ignore stellar evolution and primordial binaries for simplicity. Our results imply that most BH binary mergers take place after they get ejected from the cluster. Also, mass ratios of dynamically formed binaries should be close to one or likely to be less than 2:1. Since the binary formation efficiency is larger for higher-mass BHs, it is likely that a BH mass function sampled by gravitational-wave observations would be weighed toward higher masses than the mass function of single BHs for a dynamically formed population. Applying conservative assumptions regarding globular cluster populations such as small BH mass fraction and no primordial binaries, the merger rate of BH binaries originated from globular clusters is estimated to be at least 6.5 yr −1 Gpc −3 . Actual rate can be up to more than several times of our conservative estimate.

Merger history of primordial black-hole binaries

“…The progenitors of these BBHs are still unknown and under intensively investigation (see e.g. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]). These LIGO/Virgo BBHs present a much heavier mass distribution (in particular the sourceframe primary mass of GW170729 event can be as heavy as 50.2 +16.2 −10.2 M [7]) than that inferred from X-ray observations [28][29][30][31], which would challenge the formation and evolution mechanisms of astrophysical black holes.…”

Gravitational-wave emission from binary black holes formed in open clusters

“…The core of globular clusters (10 5 -10 6 M ⊙ ) has long been investigated as a formation site of BBHs in many previous works (e.g. Portegies Zwart & McMillan 2000;O'Leary et al 2006;Sadowski et al 2008;Downing et al 2010Downing et al , 2011Banerjee et al 2010;Tanikawa 2013;Bae et al 2014;Rodriguez et al 2015Rodriguez et al , 2016Fujii et al 2017;Park et al 2017;Askar et al 2017;Hong et al 2018;Zevin et al 2019).…”

First survey of spinning eccentric black hole mergers: Numerical relativity simulations, hybrid waveforms, and parameter estimation

“…4, where the absolute value of the relative error between the hybrid and NR amplitude and phase are shown. The quantity ∆X is defined as 22 . The errors in the amplitude are larger than the ones in the phase, but they are below the 1% threshold.…”

“…Recently, population synthesis studies [21][22][23][24] have shown that active galactic nuclei and globular clusters can host a population of moderate and highly eccentric binaries emitting in the frequency band of ground-based detectors. Therefore, the increase in sensitivity of the detectors will increase the likelihood to detect binary systems with non-negligible eccentricities.…”