A recent analysis indicates that some objects detected by their gravitational waves already come from a previous collision. These "second-generation" black holes could then form new pairs with other black holes and merge in their turn.
The study is based on 259 black hole mergers listed in the GWTC-5 catalog from the LIGO, Virgo and KAGRA collaborations. These observatories do not see the stars directly. They measure the weak deformations of spacetime produced when two very massive objects orbit each other and then merge.
When two black holes collide and merge, they release gravitational waves. These waves are detected by the LIGO-Virgo-KAGRA observatories on Earth, allowing scientists to determine the mass and spin of the black holes.
Credit: Maggie Chiang for Simons Foundation
The researchers separated this population into two statistical groups. The first group mainly includes black holes of moderate mass and low spin. The second includes heavier objects, whose spin is generally faster. This difference could preserve the trace of their history.
When two black holes merge, the final object retains most of their mass and receives a significant spin. If it remains in a dense environment, such as a star cluster, it can encounter another black hole. A new pair then forms, before another eventual merger.
According to the authors, the mass distribution of fast-spinning black holes resembles that obtained by calculating the masses of the merger products of the first group. The peaks observed in both distributions would coincide. This correspondence is presented as evidence in favor of these successive mergers.
This interpretation would also help to understand some black holes in a mass range difficult to produce directly by the collapse of a massive star. It does not completely exclude other mechanisms: matter accretion can also increase the mass and spin of a black hole.
However, the result should be considered with caution. The article is a preprint and has not yet been peer-reviewed. Future detections, as well as independent analyses, will need to confirm the actual share of these hierarchical mergers in the observed black hole population.