Astrophysicists from the California Institute of Technology (Caltech), led by Katerina Chatziioannou, analyzed data from the LIGO-Virgo-KAGRA network of gravitational-wave observatories. LIGO (Laser Interferometer Gravitational-wave Observatory) is a laser interferometer that detects gravitational waves, that is, oscillations of space-time. The network brings together detectors in the United States, Italy and Japan; over ten years it has recorded more than 300 black hole collisions. The work was published in the journal Physical Review D.
The scientists studied the mass distribution of 257 black hole mergers. It turned out that three quarters of them have a mass of about 10 solar masses. In addition, a noticeable deficit of black holes with masses from 12 to 16 solar masses was found for the first time. "We keep seeing black holes merging with a mass of 10 suns," says the first author of the work, Isaac Legred. "But why are, say, black holes with a mass of 14 suns so rare? The dip in the mass distribution indicates that producing the abundance of ten-solar-mass black holes may require a special cocktail of internal physics."
The researchers also analyzed the spin rates of black holes, which depend on how they formed. It turned out that ten-solar-mass black holes have a narrower spread of spin rates than the entire sample. Taken together, these results are consistent with the hypothesis that such black holes represent a separate population with a common origin. They may have arisen as a result of the quiet collapse of massive stars - so-called failed supernovae, when a star does not shed its envelope but immediately turns into a black hole.
However, the authors emphasize that there is still no direct evidence of failed supernovae. What is meant is that gravitational-wave observations are compatible with theoretical predictions for such a population. "We can look into the sky and study historical records of massive stars that did not explode, but this is difficult because they simply disappear without a flash, leaving a dark spot in the sky," Legred explains. "Our study shows that gravitational-wave observation data are consistent with most binary black holes forming as a result of a special kind of failed supernova, but further research is needed."
The scientists note that black holes are born in some of the most extreme conditions in the Universe, and the opportunity to better understand their formation is of great interest. For now, the conclusions remain statistical: they point to a connection between merger parameters and a possible birth scenario, but do not prove the cause.