Resolving the Stellar-Collapse and Hierarchical-Merger Origins of the Coalescing Black Holes
Yin-Jie Li, Yuan-Zhu Wang, Shao-Peng Tang, Yi-Zhong Fan
Abstract
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 ($\mathrm{ln}\mathcal{B}$) of 7.5. One category, with a mass ranging from $\ensuremath{\sim}25{M}_{\ensuremath{\bigodot}}$ to $\ensuremath{\sim}80{M}_{\ensuremath{\bigodot}}$, is distinguished by the high spin magnitudes ($\ensuremath{\sim}0.75$) and consistent with the hierarchical merger origin. The other category, characterized by low spins, has a sharp mass cutoff at $\ensuremath{\sim}40{M}_{\ensuremath{\bigodot}}$, 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 ${0.46}_{\ensuremath{-}0.24}^{+0.61}\text{ }\text{ }{\mathrm{Gpc}}^{\ensuremath{-}3}\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$. 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 $\mathrm{ln}\mathcal{B}$ of $\ensuremath{-}6.3$, suggesting that the isolated field evolution channels are contributing to the total population.