Stellar-mass black holes in young massive and open stellar clusters and their role in gravitational-wave generation III: dissecting black hole dynamics

Stellar-remnant black holes (BH) in dense stellar clusters comprise a natural setup to trigger general-relativistic (GR) inspiral and merger of binary black holes (BBH), detectable by the LISA and the LIGO-Virgo, through dynamical encounters inside such environments. In this work, the intricacies of such dynamical interactions are probed utilizing realistic, self-consistent, post-Newtonian, direct N-body evolutionary models of young massive and open stellar clusters. Particularly, the configurations of the compact subsystems, that drive the in-cluster GR BBH coalescences, are tracked on the fly. Such an approach reveals that the GR coalescences within the open clusters take place primarily via chaotic interactions involving triple BH systems. Although less frequently, such mergers are found to happen also in higher-order subsystems such as quadruples and in subsystems involving non-BH members; the mergers can themselves be BH - non-BH, which events would leave electromagnetic signatures. Close, fly-by encounters inside the clusters can also make BBHs and other types of double-compact binaries temporarily post-Newtonian; such binaries would potentially contribute to the GW background for the LISA and the PTA. These calculations, furthermore, suggest that open clusters are potential hosts for not only detached BH - main-sequence binaries, as recently identified in the globular cluster NGC 3201, but also a wide variety of other types of remnant - non-remnant binaries, which are assembled via dynamical interactions inside the clusters and which have the prospects of being discovered in radial-velocity surveys.