The branching ratio of LIGO binary black holes

Abstract: Formation of binary black holes (BBHs) detected by gravitational-wave (GW) observations could be broadly divided into two categories: those formed through field binary evolution and those assembled dynamically in dense stellar systems. The branching ratio of the BBHs refers to the contribution of each channel. The dynamical assembly channel would predict a symmetric distribution in the effective spins of the BBHs while field formation predicts BBHs to have positive effective spins. By modeling these two populations based on their effective spin distribution we show that in the 10 BBHs detected by LIGO/Virgo the contribution of the dynamically assembled BBHs to be more than about 50\% with 90\% confidence. This result is based on the assumption that the field binaries are born with positive effective spins not restricted to have small values.