Gravitational-wave Sources from Mergers of Binary Black Holes Catalyzed by Flyby Interactions in the Field

Several scenarios were suggested for the origins of gravitational-wave (GW) sources from mergers of stellar binary black holes (BBHs). Here we propose a novel origin through catalyzed formation of GW sources from ultra-wide binaries in the field. Such binaries experience perturbations from random stellar flybys that excite their eccentricities. Once a wide binary is driven to a sufficiently small pericenter approach, GW emission becomes significant, and the binary inspirals and merges. We derive an analytic model and verify it with numerical calculation to compute the merger rate to be (fwide is the fraction of wide BH-binaries), which is a relevant contribution to the observationally inferred rate. The observational signatures from this channel include spin-orbit misalignment; preference for high mass ratio BBH; preference for high velocity dispersion host galaxies; and a uniform delay-time distribution.