Intermediate-mass black hole incubators in galactic central regions

When massive stars die, their stellar corpses are often black holes weighing between 5 and 100 masses of our Sun. These are stellar-mass black holes. Although such black holes cannot be observed directly, many are known by the light they emit because they feed on their companion stars. The centres of massive galaxies are known to harbour super-massive black holes that weigh more than 100000 masses of our Sun. For example, our Milky Way holds a black hole of about 4 million masses of our Sun, while the M87 giant elliptical galaxy has one of a few billion masses of our Sun. But what about the black holes „in-between“? Do such „little-massive – big-dwarf black holes“, referred to by astronomers as „intermediate-mass black holes“, even exist?. These would be as heavy as about 1000 to 100000 masses of our Sun. Computer simulations of globular star clusters have shown that stellar-mass black holes might perhaps grow to such masses over billions of years through the mergers with other stellar-mass black holes and the swallowing of stars. But no observations have firmly confirmed their existence, and the calculations are very uncertain.

With the currently published Letter in the journal Astronomy and Astrophysics, the lead author Jaroslav Haas at the Astronomical Institute of Charles University has shown how intermediate-mass black holes probably form rapidly and often in the innermost regions of galaxies like our Milky Way. Jaroslav Haas applied calculations published a few years ago by Pavel Kroupa and collaborators of how super-massive black holes form near the centres of massive galaxies. Jaroslav Haas explains: „The innermost region of the Milky Way is forming stars all the time. Many of these are born in dense massive star clusters. When the massive stars in these clusters die they leave many stellar-mass black holes that accumulate near the central regions of the clusters. These clusters keep orbiting the centre of the Milky Way, and they often encounter dense gas clouds. When the gas falls into the cluster, the central sub-cluster of stellar-mass black holes shrinks because the black holes are slowed down through the gas. Under certain conditions the black hole sub-cluster can become so compact that the individual black holes begin to radiate gravitational waves as they orbit within the sub-cluster of stellar-mass black holes. When this happens, the sub-cluster of stellar-mass black holes can collapse catastrophically, and form an intermediate-mass black hole.“

The mass of such a collapsed cluster of black holes lies in the range 1000 to 100000 masses of our Sun. Pavel Kroupa, also from the Astronomical Institute at Charles University and professor at the University of Bonn in Germany and a co-author of this study, says: „Until now I was very doubtful that intermediate-mass black holes can exist because our previous calculations excluded their formation at the centres of galaxies. So I found this novel application of the theory of how super-massive black holes form rapidly in the early Universe remarkable. It appears very natural that many intermediate-mass black holes should orbit in the innermost region of the Milky Way and similar galaxies. Elliptical galaxies cannot be forming intermediate-mass black holes in this way now because they are not forming star clusters containing massive stars any longer.“ Sergij Mazurenko, an under-graduate student of physics at the University of Bonn in Germany and the third author of this work, exclaims: „I was working on the model formulated by Pavel Kroupa and his collaborators of how super-massive black holes form when Pavel approached me asking if I could quickly apply the model to the ideas of Jaroslav Haas. My calculations beautifully confirmed Jaroslav's first estimates.“

If this theory is correct, then the Milky would currently hold dozens of intermediate-mass black holes within about 600 light years of its central super-massive black hole. Interestingly, about 5 candidates have been reported through observations of motions of stars and gas around them. The newly published study thus brings an entirely new understanding of the dramatic and violent innermost regions of galaxies similar to our Milky Way. Jaroslav Haas concludes: „We expect new observational approaches to be undertaken now to check-up on the currently known intermediate-mass black hole candidates. And, new more realistic simulations of the evolution of massive star clusters orbiting about the centre of our Milky Way including its dense gas environment will be necessary to refine these ideas.“ Pavel Kroupa adds: „This opens up new exciting observational and computational projects for students and young researchers.“