Black hole mergers from quadruples

With the hundreds of merging binary black hole (BH) signals expected to be detected by Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo, Laser Interferometer Space Antenna (LISA), and other instruments in the next few years, the modelling of astrophysical channels that lead to the formation of compact object binaries has become of fundamental importance. In this paper, we carry out a systematic statistical study of quadruple BHs consisting of two binaries in orbit around their centre of mass, by means of high-precision direct N-body simulations including post-Newtonian (PN) terms up to 2.5PN order. We found that most merging systems have high initial inclinations and the distributions peak at ~90° as for triples, but with a more prominent broad distribution tail. We show that BHs merging through this channel have a significant eccentricity in the LIGO band, typically much larger than BHs merging in isolated binaries and in binaries ejected from star clusters, but comparable to that of merging binaries formed via the gravitational wave capture scenario in clusters, mergers in hierarchical triples, or BH binaries orbiting intermediate-mass BHs in star clusters. We show that the merger fraction can be up to ~3.4× higher for quadruples than for triples. Thus even if the number of quadruples is 20.25 per cent of the number of triples, the quadruple scenario can represent an important contribution to the events observed by LIGO/Virgo.