When massive stars die, their stellar remnants are often black holes with a size between five and 100 solar masses. Such black holes cannot be observed directly. Many are known because of the light they emit as they feed on their companion stars. The centers of massive galaxies harbor supermassive black holes weighing more than 100,000 solar masses. Our Milky Way, for example, has a black hole with about 4 million solar masses, while the giant elliptical galaxy M87 has one with several billion solar masses.
Existence not clearly confirmed
But what about the black holes “in between”? Do such “intermediate-mass black holes” even exist? These would weigh about 1,000 to 100,000 solar masses. Computer simulations of globular clusters have shown that stellar black holes could potentially grow to such masses over billions of years through mergers with other stellar black holes and the ingestion of stellar matter. However, there are no observations that clearly confirm their existence—and the calculations are very inaccurate.
Lead author Dr. Jaroslav Haas from the Astronomical Institute of Charles University has shown how medium-mass black holes are likely to form quickly and frequently in the innermost regions of galaxies such as our Milky Way. The scientist applied calculations published several years ago by a team led by Prof. Dr. Pavel Kroupa from Charles University and the Helmholtz Institute for Radiation and Nuclear Physics at the University of Bonn on the formation of supermassive black holes near the centers of massive galaxies.
“Stars are constantly forming in the innermost region of the Milky Way – many of them in dense, massive star clusters,” says Jaroslav Haas. When these massive stars die, they leave behind many stellar black holes, which accumulate near the central regions of the clusters and continue to orbit around the center of the Milky Way. In the process, these clusters often encounter dense gas clouds. “When these clouds fall into the clusters, the central subcluster of stellar black holes shrinks because the black holes are slowed down by the gas,” says Haas.
Under certain conditions, the subcluster can become so compact that individual black holes emit gravitational waves as they move within the subcluster of black holes. The subcluster of stellar black holes can then collapse catastrophically due to the emission of gravitational waves. The mass of such a collapsed cluster of black holes ranges from 1,000 to 100,000 solar masses.
“Until now, I had serious doubts that intermediate-mass black holes could exist, as our previous calculations ruled out their formation in the centers of galaxies,” says co-author Prof. Pavel Kroupa from Charles University and the University of Bonn. “That's why I found this novel application of the theory of how supermassive black holes form rapidly in the early universe remarkable.” It seems very natural that many intermediate-mass black holes should orbit in the innermost region of the Milky Way and similar galaxies. “Elliptical galaxies cannot currently develop these because they no longer form clusters of massive stars,” says the scientist, who is also a member of the transdisciplinary research areas “Modeling” and “Matter” at the University of Bonn.
Model of the formation of supermassive black holes as a basis
Sergij Mazurenko, a physics student at the University of Bonn and third author of this paper, was working on the model formulated by Pavel Kroupa and his colleagues on the formation of supermassive black holes when Kroupa asked him if he could quickly apply the model to Jaroslav Haas' ideas. “My calculations confirmed Jaroslav's initial estimates,” says Mazurenko.
If this theory is correct, then the Milky Way would currently harbor dozens of intermediate-mass black holes within a radius of about 600 light-years around its central supermassive black hole. Interestingly, observations of the movements of stars and gas around them have reported about five candidates. According to the researchers, this study brings a whole new understanding of the dramatic and violent innermost regions of galaxies similar to our Milky Way.
More realistic simulations needed
“We expect that new observational approaches will now be pursued to verify the currently known candidates for intermediate-mass black holes,” says Jaroslav Haas. To refine these ideas, new, more realistic simulations of the evolution of massive star clusters orbiting the center of our Milky Way, including their dense gas environment, will be needed. Pavel Kroupa adds, “This opens up exciting new observational and computational projects for students and young researchers.”