Heavy Black Holes Reveal Hierarchical Mergers in Dense Stellar Clusters

A new analysis of the gravitational-wave catalog compiled by the three leading observatories has uncovered two statistically separate groups of merging black holes. Objects with masses up to roughly 40 solar masses exhibit small, aligned spins, matching expectations for black holes formed directly from stellar collapse. Above about 45 solar masses, a second group appears: black holes with higher masses, rapid spin rates, and spin directions that are randomly oriented. The spin pattern of this heavier group is consistent only with objects that have already participated in one or more prior mergers.

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Laser-interferometer detectors measure minute distortions of space-time produced when compact objects collide. The first such detection in 2015 confirmed a black-hole merger and opened a new observational window. Each subsequent signal has refined knowledge of black-hole demographics, revealing that mergers occur far more often than early models predicted. Prior to this study, black holes exceeding ~45 solar masses were considered “impossible” because standard stellar-evolution physics cannot produce them.

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The study, published this month in *Nature Astronomy*, was led by a collaboration that includes Isobel M. Romero-Shaw, a coauthor based at Cardiff University. The analysis draws on data from the world’s three premier gravitational-wave facilities, which together provided 153 reliable merger detections for the catalog.

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• Total detections examined: 153 reliable black-hole merger events.

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• Heavy-object subset: 34 events classified as particularly massive.

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• Lighter population: Masses <= 40 M?, small aligned spins.

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• Heavier population: Masses >= 45 M?, rapid spins with chaotic orientations.

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The statistical separation of spin magnitude and direction between the two groups forms the key signature of hierarchical merging.

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The results indicate that the most massive black holes are not born directly from collapsing stars but are assembled through successive mergers within dense stellar environments such as globular clusters. This hierarchical growth pathway expands the known channels of black-hole formation and informs models of cluster dynamics, gravitational-wave source rates, and the overall mass spectrum of black holes in the universe.

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Romero-Shaw explained that the observed spin distribution aligns with theoretical expectations for black holes that have undergone previous mergers in densely populated clusters, providing a concrete observational signature of hierarchical assembly.

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While the gravitational-wave data reveal the masses and spins of these “impossible” black holes, they have not been detected in electromagnetic bands (X-ray or visible light). The lack of a direct electromagnetic counterpart leaves a gap in multi-messenger confirmation of their existence.

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> “This is the exact signature you would expect if black holes repeatedly merged into dense stellar clusters,” — Isobel M. Romero-Shaw, Coauthor, Cardiff University

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Ongoing observations by the three observatories will continue to add merger events to the catalog, allowing researchers to refine the statistical separation of black-hole populations and to test hierarchical-merger models with greater precision.

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