Black holes regularly collide and merge in the Universe. Scientists analyzed hundreds of such events and noticed a strange pattern. The most massive black holes formed from the merger of smaller ones in the past.
Hidden information
Whenever two black holes spiral closer together and merge, they generate gravitational waves—fluctuations in space-time. These signals contain information about the masses of both objects, as well as their spin—the speed and direction of rotation.
Pairs of black holes form in two ways. In the first case, two massive stars are born as a binary system, evolve together, and after the collapse of both components, leave behind a pair of black holes that continue to orbit each other. In the second, the black holes form independently and later capture each other during gravitational interactions in dense star clusters.
The mass of the black holes, their rotation speed, and the relative orientation of their spins depend on how it formed. That is why both scenarios leave slightly different imprints in the gravitational wave signals recorded by the LIGO, Virgo, and KAGRA observatories. However, it is only now that these patterns have been identified among the hundreds of recorded events.
Two paths to one conclusion
The first study was prepared by a group led by Katelyn Plunkett of the Massachusetts Institute of Technology. The researchers built a model based on two well-measured spin parameters that show how closely a black hole’s rotation is aligned with its orbital motion.
The second study was led by Sharan Banagiri of Monash University in Australia. His team made no prior assumptions and instead allowed statistical analysis to determine on its own how many groups the black holes should be divided into. Both papers were published in the peer-reviewed journal Physical Review Letters, according to Phys.org.
A supermassive population
Despite their different starting points, both teams identified a group of black holes distinguished by unusually large mass. Each such object is at least 40 times more massive than the Sun.
According to Katelyn Plunkett’s conclusions, these giants have fast and chaotically oriented spins. This configuration is characteristic of black holes that formed as a result of earlier mergers, rather than through the collapse of stars.
Sharan Banagiri and his colleagues arrived at a similar threshold value and also detected high spins. However, their analysis did not find a clear sign of origin from earlier collisions, so the authors urge caution in interpretation.
The second generation
Stellar collapse does not create a black hole in the range of roughly 60 to 130 solar masses because of the effect of pair instability, so the supermassive group with chaotic spins points to another path for the formation of such objects.
Together, these results are among the strongest evidence to date for the existence of second-generation black holes. This is what scientists call objects that did not arise from the collapse of a massive star, but from the merger of two predecessors.
This also suggests a possible path for the formation of the supermassive black holes at the centers of galaxies. A chain of repeated mergers could gradually increase their mass step by step.
