Astronomers find evidence for three subpopulations of merging black holes - Astronomy & Space - Astronomy - UNIVERSE

Artist's impression of a pair of black holes merging, involving one with unusual spin. Credit: Carl Knox, OzGrav, Swinburne University of Technology

Astronomers analyzing gravitational-wave data from the LIGO-Virgo-KAGRA Collaboration have reported that merging binary black holes fall into three distinct categories. The study shows that the three subpopulations have their own characteristic masses, spin behavior, and merger rate that may be linked to different dominant formation mechanisms. The paper outlining their results was submitted to the preprint server arXiv on March 18.

A mix of three

The data in the fourth gravitational-wave catalog (GWTC-4), released by the LIGO-Virgo-KAGRA collaboration, included more than 150 detected black hole mergers. The analysis of this dataset revealed that the overall population of merging binary black holes may not have the same origins.

When researchers analyzed how the masses of black holes are distributed across the population, they saw prominent peaks around 10 solar masses and 35 solar masses. Similar features were also seen in how the systems' spins and mass ratios behave, with noticeable changes around 20 and 40 solar masses. If the same process drove all mergers, a smoother distribution would be expected. These features suggested that black hole mergers may be produced by multiple formation channels.

In the new study, researchers simulated key properties, such as masses, spin behavior, and merger rates, to reproduce the observed features of the overall population, and found that it is best explained as a mixture of three distinct groups of binary black holes. They then linked the parameter values that represent the properties of each group with theoretical predictions to identify the most likely pathway that formed them.

Distributions of primary masses of the three simulated components: first (in blue), second (yellow), and third (green) subpopulations. The first subpopulation shows a peak around 10 solar masses and the second one shows a peak around 35 solar masses. Credit: arXiv (2026). DOI: 10.48550/arxiv.2603.17987

Heavy, heavier, heaviest

The first group of binary black holes, which makes up 79% of the overall population, shows a sharp peak around 10 solar masses. These low-mass black holes are slowly spinning systems with very little wobbling. Their spins are also aligned with the orbit.

All these features point to the fact that these binary black holes likely originated through an isolated binary evolution. That is, when two stars born as a pair evolve, exchange mass, and collapse into black holes that merge, without external influence.

The second subpopulation of binary black holes makes up nearly 14.5% of all detected binaries and it explains the prominent feature around 35 solar masses seen in the observations.

The team found that these binaries have black holes of nearly equal masses and equal fractions of black holes' spins aligned and misaligned with the orbit, with greater wobbling compared to the first group. These intermediate-mass black holes show signs of a more chaotic origin compared to the first group.

Researchers suggest that these binaries likely formed in a crowded environment, such as a globular cluster. They say that a pair of black holes influenced by a third distant object could also produce these binary black holes.

Finally, the third population, which makes up only 2.5% of the overall population, falls at the higher end of the mass distribution. These systems have black holes of unequal mass and show complex spin behavior, with noticeable wobbling. Researchers suggest that they likely formed through hierarchical mergers, with at least one of the black holes being a remnant of an earlier merger.

However, researchers note that these formation channels are what likely dominated each subpopulation, but there may be other processes at work.

"While these conclusions are reasonably robust, the direct association of subpopulations with single channels remains elusive," they write in their paper. With the upcoming data releases of the LIGO-Virgo-KAGRA collaboration, they aim to produce more conclusive results about how these different merging black hole populations form. 

by Shreejaya Karantha, Phys.org

edited by Sadie Harley, reviewed by Robert Egan

Source: Astronomers find evidence for three subpopulations of merging black holes