Resolving the Stellar-Collapse and Hierarchical-Merger Origins of the Coalescing Black Holes

Spin and mass properties provide essential clues in distinguishing the origins of coalescing black holes (BHs). With a dedicated semiparametric population model for the coalescing binary black holes (BBHs), we identify two distinct categories of BHs among the GWTC-3 events, which is favored over the one population scenario by a logarithmic Bayes factor (lnB) of 7. 5. One category, with a mass ranging from ∼25M⊙ to ∼80M⊙, is distinguished by the high spin magnitudes (∼0. 75) and consistent with the hierarchical merger origin. The other category, characterized by low spins, has a sharp mass cutoff at ∼40M⊙, which is natural for the stellar-collapse origin and in particular the pair-instability explosion of massive stars. We infer the local hierarchical merger rate density as 0. 46-₀. ₂₄^+0. 61 Gpc^-3 yr^-1. Additionally, we find that a fraction of the BBHs has a cosine-spin-tilt-angle distribution concentrated preferentially around 1, and the fully isotropic distribution for spin orientation is disfavored by a lnB of -6. 3, suggesting that the isolated field evolution channels are contributing to the total population.