Mass and spin properties of black-hole mergers reveal formation in triples

Deciphering the formation channels of observed binary black hole mergers remains a central open problem in gravitational-wave astronomy. With hundreds of detections, the inferred distributions of masses and spins show a rich diversity that is difficult to reconcile with traditional formation scenarios involving isolated binary stars or dense stellar environments. The authors focus on the large fraction of massive progenitor stars found in hierarchical triples.

They investigate black hole mergers driven by gravitational perturbations from tertiary companions, using simulations of the systems' dynamics and stellar evolution.

The simulations reproduce key features of the observed merger population: a sharp primary mass peak at m1 ~ 10 solar masses that dominates the population; substantial fractions of systems with large spin-orbit angles both within and beyond that peak, matching the skewed distributions of the spin parameters chi_eff and chi_p; and a mass-ratio distribution favoring equal masses with matching slopes. The models further predict a sharp decline of mergers near m1 ~ 30 solar masses, coincident with the location of inferred features that may signal another channel dominating the high-mass tail.

These models of hierarchical triples demonstrate, for the first time, a formation scenario that simultaneously reproduces the principal properties inferred for the low-mass bulk of binary black hole mergers.