A Triple Origin for the Heavy and Low-Spin Binary Black Holes Detected by LIGO/Virgo

Abstract

We explore the masses, merger rates, eccentricities, and spins for field binary black holes driven to merger by a third companion through the Lidov-Kozai mechanism. Using a population synthesis approach, we model the creation of stellar-mass black hole triples across a range of different initial conditions and stellar metallicities. We find that the production of triple-mediated mergers is enhanced at low metallicities by a factor of ~100 due to the lower black hole natal kicks and reduced stellar mass loss. These triples naturally yield heavy binary black holes with near-zero effective spins, consistent with most of the mergers observed to date. This process produces a merger rate of between 2 and 25 Gpc^-3 yr^-1 in the local universe, suggesting that the Lidov-Kozai mechanism can potentially explain all of the low-spin, heavy black hole mergers observed by Advanced LIGO/Virgo. Finally, we show that triples admit a unique eccentricity and spin distribution that will allow this model to be tested in the near future.