The Heavier the Faster: A Subpopulation of Heavy, Rapidly Spinning and Quickly Evolving Binary Black Holes

Abstract The spins of binary black holes (BBHs) measured from gravitational waves contain valuable information about their formation pathways. In this study, we propose a new quantity, the “dimensionless net spin” ( χ N ), which relates to the sum of the angular momenta of the component black holes (BHs) in the system, offering a novel perspective for exploring the origins of BBHs. Through hierarchical Bayesian inference on χ N , we find strong evidence that its distribution is better described by two Gaussian components rather than one, and their branching ratios vary with total mass and redshift: there is a narrow peak at χ N ∼ 0.15 and an extended peak at χ N ∼ 0.3–0.6. The rapidly spinning systems likely dominate the high-mass end of the population and evolve with redshift more quickly. These findings present new challenges to the field binary formation scenario and suggest that dynamical processes may play a key role in the formation of high total mass BBHs.