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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

  • *Contact author: sharan.banagiri@monash.edu
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Phys. Rev. Lett. 137, 021403 – Published 6 July, 2026

DOI: https://doi.org/10.1103/blyb-lqv6

Abstract

With the release of the fourth LIGO-Virgo-KAGRA gravitational-wave catalog (GWTC-4), we are starting to gain a detailed view of the population of merging binary black holes. The formation channels of these black holes is not clearly understood, but different formation mechanisms may lead to subpopulations with different properties visible in gravitational-wave data. Adopting a phenomenological approach, we find GWTC-4.0 data supports the presence of at least three subpopulations, each associated with a different range of black hole mass and with sharp transition boundaries between them. Each subpopulation is characterized by different distributions for either the mass ratios, the black-hole spin magnitudes or both. Subpopulation A with primary mass m1≤27.7−3.4+4.1M⊙ (90% credibility), is characterized by a nearly flat mass ratio distribution q=m2/m1, and by small spin magnitudes (χ≤0.5−0.1+0.1). Subpopulation B, with 27.7−3.4+4.1M⊙≤m1≤40.2−3.2+4.7M⊙, has a much sharper preference for mass ratio q≈1. Subpopulation C, with m1≥40.2−3.2+4.7M⊙, has support for large spin magnitudes, and tentative support for mass ratios q≈0.5. We interpret these transitions as evidence for multiple subpopulations, each potentially associated with a different formation pathways. We suggest putative formation scenarios for each subpopulation, and explore chemically homogeneous evolution, population III stars and dynamical formation channels as an explanation for subpopulation B. Our findings for subpopulation C are largely consistent with recent claims of hierarchical mergers but with some curious differences in properties.

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

Cailin Plunkett, Salvatore Vitale, Thomas Callister, and Michael Zevin (Society of Physicists Interested in Non-Aligned Spins (SPINS))
Phys. Rev. Lett. 137, 021404 (2026)

Article Text

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Outline

Information

Phys. Rev. Lett. 137, 021403– Published 6 July, 2026
  • Received 29 September 2025
  • Revised 18 February 2026
  • Accepted 8 May 2026
Article part of CHORUS
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