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 ($\ln \mathcal{B}$) of 7.5. One category, with a mass ranging from $\sim 25M_{\odot }$ to $\sim 80M_{\odot }$, is distinguished by the high spin magnitudes ($\sim 0.75$) and consistent with the hierarchical merger origin. The other category, characterized by low spins, has a sharp mass cutoff at $\sim 40M_{\odot }$, 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 $0.46_{-0.24}^{+0.61}\mathrm{G}\mathrm{p}{\mathrm{c}}^{-3}\mathrm{y}{\mathrm{r}}^{-1}$. 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 $\ln \mathcal{B}$ of -6.3, suggesting that the isolated field evolution channels are contributing to the total population.