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 semi-parametric 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^+0.61_-0.24  Gpc^-3yr^-1. Additionally, we find that a fraction of the BBHs has a cosine-spin-tilt-angle distribution concentrated preferentially around 1, which is consistent with the expectation of the isolated field binaries and the dynamically formed BBHs in the accretion disks of Active Galactic Nuclei.