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Good things always come in 3s: Trimodality in the binary black-hole chirp-mass distribution supports bimodal black-hole formation

Year: 2026
Type: article
Abstract: The latest GWTC-5 release from the LIGO–Virgo–KAGRA (LVK) collaboration builds upon the known population of double compact object mergers and reveals new structures in the chirp-mass distribution of merging binary black holes (BBHs). Below 30 M ⊙ , the chirp-mass structure is roughly trimodal, with a sharp drop at ∼10 M ⊙ and a more tentative dip at ∼17 M ⊙ ; we argue that these features are well explained by recent, detailed stellar evolution models, which predict a bimodal distribution in the birth masses of black holes (BHs). This predicted distribution depends only weakly on metallicity and naturally extends to a trimodality in chirp mass, implying universal structural features that can be tested with LVK observations. Using a new compact-remnant mass prescription derived from these models, we performed rapid population synthesis simulations to test the robustness of the predicted chirp-mass structure against uncertainties in binary evolution and cosmic star formation history and compared these simulations with the current observational data. The trimodal chirp-mass distribution emerges as a robust outcome of the new remnant-mass model, persisting across adopted variations in binary and cosmic physics. In contrast, traditional BH formation models lacking a bimodal BH mass spectrum fail to reproduce the observed structure, especially around the sharp drop at ∼10 M ⊙ . Intriguingly, the central chirp-mass peak, dominated by unequal-mass BBHs, originates from a previously underappreciated formation pathway in which strong luminous blue variable winds suppress any binary interaction before the first BH forms. If isolated binary evolution dominates BBH formation below 30 M ⊙ , these universal structure features may provide powerful observational constraints on BH formation, binary evolution, and cosmic star formation, and they may also serve as standard sirens for precision cosmology.
DOI: 10.1051/0004-6361/202557611
Authors: R. Willcox, F. R. N. Schneider, E. Laplace, P. Podsiadlowski, K. Maltsev, I. Mandel, P. Marchant, H. Sana, T. G. F. Li, T. Hertog
Institutions: KU Leuven, Heidelberg University, Heidelberg Institute for Theoretical Studies, University of Amsterdam, University of Oxford, Max Planck Institute for Astronomy, ARC Centre of Excellence for Gravitational Wave Discovery, Monash University, Ghent University
Language: English
Open Access status: gold
Estimated APC paid: $1,865
Funders: European Commission, Deutsche Forschungsgemeinschaft, Fonds Wetenschappelijk Onderzoek, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, KU Leuven, Onderzoeksraad, KU Leuven, Klaus Tschira Stiftung, European Research Council, Australian Research Council
Awards: G1620761618, Aiming for the massive stars: understanding the progenitors of black holes and supernovae in the Gaia, LIGO, and LSST era, G2997735311, G3681572691, G4299867415, G6526081583, ARC Centre of Excellence for Gravitational Wave Discovery, G675796426, EXC 2181: STRUKTUREN: Emergenz in Natur, Mathematik und komplexen Daten, G7227535049, G74030750, G7545584363, Understanding mixing and chemical enrichment in massive stars, G8524619422
OpenAlex ID: W7204604879
FWCI: 18.89
Cites: 164
Cited by: 23
Related to: 10