Distinct spin properties and astrophysical origin of low mass binary black holes in gravitational wave data

We analyze the effective-spin distribution of binary black hole mergers in GWTC-5.0 as a function of primary black hole mass using hierarchical Bayesian inference. We model the population as a mixture of two spin components separated by a transition mass scale inferred directly from the data. We find strong evidence for a transition at $\tilde{m} = 15.2^{+4.3}_{-3.6}\, M_\odot$. Mock-catalog analyses show that such a transition is unlikely to arise from finite-sample fluctuations of a mass-independent $\chi_{\rm eff}$ population and the posterior predictive distributions of $\chi_{\rm eff}$ inferred below and above the transition are clearly distinct. Below the transition mass, the effective-spin distribution is narrow, peaks at a small positive value $\chi_{\rm eff}>0$, but also shows significant support for negative $\chi_{\rm eff}$. Above the transition, the distribution is broader and its peak shifts to values consistent with $\chi_{\rm eff}\simeq0$, making its support at both positive and negative $\chi_{\rm eff}$ roughly similar. These findings suggest that the dominant merger population concentrated around $10\,M_{\odot}$ is statistically distinct from the rest and that it arises from a different formation channel. We show that this low-mass population is broadly consistent with formation from massive stellar multiples in the field: it may either arise from isolated binary star evolution but only if black hole natal kicks below $\tilde{m}$ are generally very large ($\gtrsim100\,\rm km/s$) or be caused by the dynamical evolution of hierarchical triples. In contrast, isolated binary evolution with standard fallback kick models cannot reproduce the support for negative $\chi_{\rm eff}$.