Formation and Merging of Mass Gap Black Holes in Gravitational-wave Merger Events from Wide Hierarchical Quadruple Systems

We investigate secular evolution in hierarchical quadruple systems as a formation channel of mass-gap black holes (with masses of about 3-5M) in systems that will eventually lead to binary black hole mergers detectable by ground-based gravitational-wave detectors (LIGO/Virgo). We show that in a 3 + 1 hierarchical system, two episodes of induced mergers would first cause two neutron stars to merge and form a mass-gap black hole, which will subsequently merge with another (more massive) black hole through a second induced merger. We demonstrate that such systems are stable to flybys, and their formation would predict a high mass ratio and eccentric merger of a mass-gap black hole with a more massive black hole companion. Such a formation channel may explain observed gravitational-wave events such as the recently discovered LIGO/Virgo events S190814bv and S190924h.