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Signatures of a Subpopulation of Hierarchical Mergers in the GWTC-4 Gravitational-Wave Dataset

Cailin Plunkett* and Salvatore Vitale

Thomas Callister

Michael Zevin

  • LIGO Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

  • Adler Planetarium, 1300 South DuSable Lake Shore Drive, Chicago, Illinois 60605, USA, Center for Interdisciplinary Explorationand Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Avenue, Evanston, Illinois 60201, USA, and NSF-Simons AI Institute for the Sky (SkAI), 172 East Chestnut Street, Chicago, Illinois 60611, USA

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Phys. Rev. Lett. 137, 021404 – Published 6 July, 2026

DOI: https://doi.org/10.1103/n6p4-ftgq

Abstract

Repeated black-hole mergers in dense stellar clusters are a plausible mechanism to populate the predicted gap in black hole masses due to the pair-instability supernova process. These hierarchical mergers carry distinct spin characteristics relative to first-generation black holes. We introduce an astrophysically motivated model in the joint space of effective inspiral and precessing spins, which captures the dominant spin dynamics expected for hierarchical mergers. We find decisive evidence for a transition at m1=46.2−7.2+12.6M⊙, above which the population is nearly entirely hierarchical, a location consistent with the anticipated onset of the pair-instability gap. We also infer a global peak in the hierarchical merger rate at m1=15.7−1.1+3.2M⊙. The existence of low- and high-mass subpopulations of higher-generation black holes suggests the contribution of both near-solar-metallicity and metal-poor star clusters to the hierarchical merger population. Our results reinforce the growing evidence for detailed, mass-dependent substructure in the spin distribution of the binary black hole population.

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Physics Subject Headings (PhySH)

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Evidence Mounts for Hierarchical Black Hole Mergers

Published 6 July, 2026

Different analyses of gravitational-wave observations are converging on evidence for a distinct population of massive black hole binaries produced through repeated mergers.

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Evidence for Three Subpopulations of Merging Binary Black Holes at Different Primary Masses

Sharan Banagiri, Eric Thrane, and Paul D. Lasky
Phys. Rev. Lett. 137, 021403 (2026)

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Phys. Rev. Lett. 137, 021404– Published 6 July, 2026
  • Received 12 January 2026
  • Accepted 24 April 2026
Article part of CHORUS
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