Characterizing Binary Black Hole Subpopulations in GWTC-4 with Binned Gaussian Processes: On the Origins of the 35 M ⊙ Peak

Abstract Understanding the astrophysical origins of binary black holes requires accurate and flexible modeling of multidimensional population properties. In this Letter, using a data-driven framework based on binned Gaussian processes, we characterize the joint distribution of BBH primary masses, mass ratios, and effective inspiral spins. We identify three distinct subpopulations in the GWTC-4 sample of observations and investigate their astrophysical origins. We find that only one of the three subpopulations exhibits the 35 M ⊙ peak, which is characterized by a strong preference for equal mass systems and isotropic spin orientations. Our inferred distributions are consistent with a predominantly dynamical origin of this feature. By comparing with theoretical simulations, we further show that the subpopulation that exhibits the 35 M ⊙ peak can exclusively comprise dynamically assembled systems in globular clusters, specifically if black hole birth spins are in the range (0.1−0.2), whereas the other two subpopulations require substantial contributions from alternative formation channels. We constrain the lower bound on the merger rate of BBHs in globular clusters to be 0.6 9 − 0.33 + 0.23 Gpc − 3 yr − 1 , which is consistent with most theoretical predictions (that can range from 0.2 to 57 Gpc −3 yr −1 depending on modeling assumptions). We conclude that dynamical formation in globular clusters remains a strong candidate for the origin of this excess near 30−40 M ⊙ and that more data and targeted parametric models are necessary to rigorously establish this interpretation.