Black hole and neutron star mergers in galactic nuclei: the role of triples

Abstract

Nuclear star clusters that surround supermassive black holes (SMBHs) in galactic nuclei are thought to contain large numbers of black holes (BHs) and neutron stars (NSs), a fraction of which form binaries and could merge by Kozai-Lidov oscillations (KL). Triple compact objects are likely to be present, given what is known about the multiplicity of massive stars, whose life ends either as a NS or a BH. In this paper, we present a new possible scenario for merging BHs and NSs in galactic nuclei. We study the evolution of a triple black hole (BH) or neutron star (NS) system orbiting an SMBH in a galactic nucleus by means of direct high-precision N -body simulations, including Post-Newtonian terms. We find that the four-body dynamical interactions can increase the KL angle window for mergers compared to the binary case and make BH and NS binaries merge on shorter timescales. We show that the merger fraction can be up to ∼ 5-8 times higher for triples than for binaries. Therefore, even if the triple fraction is only ∼ 10%-20% of the binary fraction, they could contribute to the merger events observed by LIGO/VIRGO in comparable numbers.

GW190412: Observation of a binary-black-hole coalescence with asymmetric masses

“…BBHs in triple or quadruple systems can undergo Lidov-Kozai oscillations [211,212] that may expedite the GW inspiral of the binary. Such systems can either exist in the galactic field with a stellar-mass outer perturber [213][214][215][216][217][218][219] or in galactic nuclei with a supermassive BH as the tertiary component [220][221][222][223]. Though most modeling of hierarchical stellar systems does not include robust predictions for mass ratio distributions of merging BBHs, certain models find galactic field triples with asymmetric masses for the inner BBH to have a merger fraction that is about twice as large as their equalmass counterparts [213].…”

Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational-Wave Event S190521g

“…The two main channels to BBH mergers are believed to be field binary star evolution, e.g., [2,3], and dynamical encounters. Dynamical mergers can occur in globular clusters [4,5], galactic nuclei [6][7][8], and in gas disks in galactic nuclei [9][10][11][12][13][14][15][16][17]. Mergers involving > 50 M ⊙ black holes (BHs) are unlikely to involve field binary stars [18].…”

Spin and eccentricity evolution in triple systems: From the Lidov-Kozai interaction to the final merger of the inner binary

“…It has been suggested that a significant amount of binary black-hole (BH) mergers detectable by Advanced LIGO (aLIGO; [1]) and Advanced Virgo (aVirgo; [2]) may happen in galactic nuclei [3][4][5] or in surrounding gas disks [6][7][8]. The recent announcement by the Zwicky Transient Facility [9] further strengthens this possibility.…”

“…[27] for a review of different formation scenarios). Different authors have investigated this problem in different context, ranging from galactic nuclei (e.g., [4,5,[28][29][30][31]), to dense stellar clusters (e.g., [32,33]), to isolated field stars (e.g., [34][35][36][37][38][39][40][41]).…”