The largest black holes in the universe may not be born that way. A new study from Cardiff University suggests they are built up over time through repeated collisions inside densely packed star clusters, a finding drawn from the most comprehensive gravitational-wave catalog ever assembled.
The research examined version 4.0 of the LIGO-Virgo-KAGRA Gravitational-Wave Transient Catalog, known as GWTC4, which contains 153 reliable detections of merging black holes. By combing through that data, the Cardiff team identified two distinct populations of black holes with markedly different characteristics, pointing toward two very different formation histories.
The lower-mass population looked familiar: black holes consistent with what scientists expect when massive stars collapse at the end of their lives. The higher-mass population was different in a specific and telling way. Those heavier black holes showed more rapid spins, oriented in seemingly random directions, exactly the pattern predicted for black holes that have already merged once before and then collided again.
"Gravitational-wave astronomy is now doing more than counting black hole mergers," said lead author Dr. Fabio Antonini from Cardiff University's School of Physics and Astronomy. "It is starting to reveal how black holes grow, where they grow, and what that tells us about the lives and deaths of massive stars. This is exciting because we can use the information to test our understanding of how stars and clusters evolve in the Universe."
The scenario the researchers describe is sometimes called hierarchical merging. In extremely dense stellar environments, where stars are packed up to a million times more tightly than in the region around our Sun, black holes formed from dying stars can collide and merge. The object that results from that merger is larger and heavier than either of its predecessors, and it can then go on to collide again. Through this chain of smashups, black holes can grow far beyond the size any single stellar collapse could produce.
The spin data was what made the two populations stand out so clearly to the research team. When a black hole forms directly from a collapsing star, its spin tends to be slow and aligned with the original star's rotation. When two black holes merge, the resulting object acquires a faster spin, and the direction of that spin reflects the geometry of the collision rather than any prior alignment. Random spin orientations in a large sample are therefore a signature of repeated merging.
"What surprised us most was how clearly the high-mass black holes stand out as a separate population," said co-author Dr. Isobel Romero-Shaw, Ernest Rutherford Fellow at Cardiff University. "Unlike the lower-mass systems we analyzed, which were generally slowly-spinning, the higher-mass systems are consistent with having more rapid spins, oriented in seemingly random directions. This is the exact sign" of hierarchical formation in dense cluster environments, the researchers said.
The findings were published in Nature Astronomy. Gravitational-wave detectors like LIGO, Virgo, and KAGRA sense the tiny ripples in spacetime produced when two massive objects spiral into each other and merge. Each detection adds a data point, and with 153 reliable events now in the catalog, patterns that would have been invisible in smaller samples are beginning to emerge.
The study does not close the question of how the universe's most massive black holes form, but it gives astronomers a concrete mechanism to investigate further, one rooted in the violent, crowded environments of dense star clusters rather than in the death of any single star.
