Study Links Heavy Black Holes to Repeated Mergers

A study published this month in Nature Astronomy examined gravitational-wave signals recorded by three observatories and identified two populations of merging black holes. Black holes up to about 40 solar masses displayed the small, aligned spins expected from single-star collapse.

Objects above approximately 45 solar masses instead showed rapid spins in random directions, a pattern produced only by repeated mergers.

The database contained 34 events involving these heavier objects.

Their spin statistics match predictions for black holes assembled through successive collisions inside dense stellar environments. Isobel M. Romero-Shaw, a coauthor at Cardiff University, said the observed signature matches expectations for repeated mergers in such clusters.

Researchers have not yet detected any of these intermediate-mass black holes directly in electromagnetic observations. Their presence is inferred solely from the gravitational-wave data. The findings indicate that the heaviest stellar-mass black holes are built through successive mergers rather than formed in a single collapse event.