A subpopulation of low-mass, spinning black holes: signatures of dynamical assembly

Abstract

Gravitational-wave observations of massive, rapidly spinning binary black holes mergers provide increasing evidence for the dynamical origin of some mergers. Previous studies have interpreted the mergers with primary mass $\gtrsim45\,M_\odot$ as being dominated by hierarchical, second-generation mergers, with rapidly spinning primaries being the products of previous black hole mergers assembled in dense stellar clusters. In this work, we reveal confident evidence of another subpopulation with rapid and isotropic spins at low mass containing the two exceptional events GW241011 and GW241110, consistent with a hierarchical merger hypothesis. Our result suggests the mass distribution of the second-generation black holes is peaked at low primary masses of $\sim16\,M_\odot$ rather than $\gtrsim45\,M_\odot$ in the pair-instability gap. Such low-mass second-generation black holes must be formed from the merger of even lighter first-generation black holes, implying that dense, metal-rich stellar environments contribute to the binary black hole population. By separating the contamination of higher-generation black holes, our result reveals the primary mass distribution of first-generation black holes formed from stellar collapse, which shows a significant dip between $\sim12\,M_\odot$ to $\sim20\,M_\odot$. This may indicate a dearth of black holes due to variation in the core compactness of the progenitor.

Figures (8)

• Figure 1: A graphical illustration of our strongly-parameterized model showing how the distribution of effective inspiral spin $\chi_\text{eff}$ varies with primary mass $m_1$. We interpret the truncated Gaussian distribution as arising from 1G+1G mergers while we attribute the uniform distribution to 2G+1G mergers.
• Figure 2: The weakly-modeled 2G+1G fraction as a function of primary mass. The result in gray uses 153 BBH detections in GWTC-4 while blue includes GW241011 and GW241110. The median posteriors and 90% credible intervals are presented. The fraction is consistent with zero for $m_1\sim90M_\odot$ and $m_1>120M_\odot$ since the lack of the detections makes the data uninformative in these mass scales.
• Figure 3: The primary mass distribution of binary black hole systems for 2G+1G mergers (blue) and 1G+1G mergers (orange). The 1G+1G rate falls off sharply near $45 M_\odot$, which we interpret as the edge of the pair instability gap. We also highlight an interesting dip in the orange 1G+1G distribution between $\sim12M_\odot-18M_\odot$.
• Figure 4: The multiple spin transition masses described in Eq. \ref{['eq:parametric_spin']}. The analysis in green uses GWTC-4 events while the results in blue and orange include GW241011 and GW241110. We impose fixed mixture fractions ($\xi_\text{low-mass}=\xi_\text{high-mass}=1$) and boundaries of the uniform components ($\chi_\text{min}^\text{low-mass}=\chi_\text{min}^\text{high-mass}=-0.47$, $\chi_\text{max}^\text{low-mass}=\chi_\text{max}^\text{high-mass}=0.47$).The result from a more flexible model where the uniform distribution boundaries for low and high mass spinning subpopulations are independent and free parameters is indistinguishable from the blue trace.
• Figure 5: The measurements of the half-width and the mean of the $\chi_{\rm{eff}}$ distribution of the low and high mass spinning subpopulations in the model where the boundaries of the uniform components are free. The means are defined as $\mu_{{\tilde{m}_{\mathrm{low,high}}}}=(\rm{max}_{{\tilde{m}_{\mathrm{low,high}}}}+\rm{min}_{{\tilde{m}_{\mathrm{low,high}}}})/2$ and the half-widths are $\rm{w}_{{\tilde{m}_{\mathrm{low,high}}}}=(\rm{max}_{{\tilde{m}_{\mathrm{low,high}}}}-\rm{min}_{{\tilde{m}_{\mathrm{low,high}}}})/2$.