Astronomers find evidence for three subpopulations of merging black holes

A new analysis of gravitational-wave data collected by the LIGO-Virgo-KAGRA Collaboration has revealed that merging binary black holes do not constitute a single, homogeneous population. Instead, they appear to fall into three distinct groups, each with its own characteristic masses, spin properties, and formation pathway. The results were submitted to the preprint server arXiv on March 18, 2026.

The research team drew on the fourth gravitational-wave catalog, known as GWTC-4, which includes more than 150 confirmed black hole merger events. By simulating key observable properties — masses, spin behavior, and merger rates — the researchers matched the data to a mixture of three statistically distinct components, suggesting that no single formation channel can explain the full population of merging black holes.

The first and most numerous subpopulation accounts for approximately 79 percent of all detected mergers. These systems cluster around a primary mass of roughly 10 solar masses, show very little spin misalignment relative to their orbital axis, and exhibit slow rotation. The authors interpret these properties as signatures of isolated binary evolution, a process in which two stars born together evolve as a gravitationally bound pair, exchange mass, and eventually collapse into black holes without any significant external perturbation.

The second subpopulation represents around 14.5 percent of the total sample and is responsible for a well-known observational feature: a pronounced clustering of black hole masses near 35 solar masses. Unlike the first group, these binaries display roughly equal fractions of aligned and misaligned spins, suggesting a more chaotic formation history. The researchers propose that these systems likely originated in crowded stellar environments such as globular clusters, where gravitational interactions between multiple objects are common, or were assembled through the influence of a third distant body.

The third and rarest subpopulation constitutes only about 2.5 percent of detections. These systems sit at the higher end of the observed mass distribution, have unequal mass ratios, and show complex precessing spin behavior. The authors attribute these properties to hierarchical mergers — events in which at least one of the black holes is itself the product of a previous merger rather than a collapsed stellar remnant. This pathway has been theorized for some time, and the new statistical analysis provides some of the clearest observational support yet for its existence.

The authors acknowledge that while the three-component model reproduces the observed data robustly, the direct causal connections between each subpopulation and a specific astrophysical channel remain probabilistic rather than definitive. Future observing runs by the LIGO-Virgo-KAGRA network are expected to expand the catalog significantly, enabling more precise constraints on the relative contributions of each formation mechanism.

Source: Ananya Ray et al. «On the Astrophysical Origin of Binary Black Hole Subpopulations: A Tale of Three Channels.» arXiv (2026). DOI: 10.48550/arXiv.2503.17987

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