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Abstract

The spins of binary black holes (BBHs) measured from gravitational waves
carry notable information of the formation pathways. Here we propose a quantity
"dimensionless net spin" ($\chi_{\rm N}$), which is related to the sum of
angular momentum of component black holes in the system, to provide a novel
perspective to study the origin(s) of BBHs. By performing hierarchical Bayesian
inference on $\chi_{\rm N}$, we find strong evidence that the marginal
distribution of this quantity can be better fitted by two Gaussian components
rather than one: there is a narrow peak at $\chi_{\rm N} \sim 0.15$ and another
extended peak at $\chi_{\rm N} \sim 0.47$. We also find that the rapidly
spinning systems likely dominate the high-mass end of the population and they
evolve with redshift much quicker. These findings bring new challenges to the
field binary scenario, and suggest that dynamical process should plays a key
role in forming high total mass BBHs.