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Black holes: the missing link in spherical mass?  – The place where mysterious intermediate black holes can form

Black holes: the missing link in spherical mass? – The place where mysterious intermediate black holes can form

Fertile breeding ground: Astronomers may have discovered where and how intermediate black holes — a previously mysterious intermediate form of such singularities — form. Accordingly, the dense centers of star clusters could be the birthplace of these “medium-sized” black holes, but only when the star clusters are still very young, the team explains in the journal Science. It is also possible that there are intermediate black holes in the vicinity of our Milky Way Galaxy.

It's the “missing link” between black holes: average black holes have a mass of about 100 to tens of thousands of times the mass of the Sun, and thus lie exactly in the gap between stellar black holes created by stellar explosions and the supermassive samples that form. In the center over hundreds of millions of years galaxies have grown. But how the intermediate form between the two arises remains a mystery.

Puzzles about the formation of mediators

One reason for this: Intermediate black holes are extremely rare, and there are only a few potential candidates. They include an object called LB-1 with a mass of about 70 solar masses, another with a mass of 428 solar masses, and an X-ray source weighing about 50,000 solar masses, whose radiation signature could indicate the presence of an active intermediate black hole.

But how did these intermediate forms arise? The lighter samples could be the result of merging stellar black holes, as gravitational wave data suggest. But there is still no definitive explanation for heavier intermediate black holes. Astronomers have only several hypotheses, including sequential mergers or spontaneous collapse of dense gas clouds directly into the black hole – similar to what is postulated for precursors of supermassive black holes in the early universe.

The Omega Centauri globular star cluster in our Milky Way galaxy could hide an intermediate black hole at its center. © ESO/INAF-VST/OmegaCAM/ CC by 4.0

Spherical clusters as fertile ground?

But there is another possible scenario: according to this, the dense center of globular clusters could be a “breeding ground” for intermediate black holes. There, massive stars are so close together that they often collide and merge with each other. If the resulting giant stars explode in a supernova, the black holes formed could also merge with each other, thus growing to an average size. Astronomers have already found evidence of clusters of stellar black holes in such globular clusters.

But the problem is that very massive stars in globular clusters lose a significant portion of their mass due to strong stellar winds before a supernova. “As a result, they evolve into black holes with a mass much less than about 1,000 solar masses,” explained Michiko Fujii of the University of Tokyo and her colleagues. But how do the larger specimens come about?

Reconstructing the evolution of each individual star

Astronomers may now have found an answer to this question. For their study, Fujii and her team examined the processes at the core of globular star clusters in more detail, focusing on the early days of these star clusters. “At this stage, the young star clusters are still enveloped in their original cloud of cold molecular gas,” they explain. This dense, heavy cocoon brings the gas and stars closer together in the middle of the young clusters.

“For the first time, we have now succeeded in using numerical simulations to understand how individual stars evolve at this early stage of globular clusters,” says Fujii. Until now, the computational effort was too high for this, so the team first had to develop a new type of simulation software. “By tracking individual stars with realistic masses, we can also reconstruct their collisions in this environment,” says the astronomer.

Mass accumulation in the center of the pile

Simulations revealed that young globular clusters could certainly become breeding grounds for intermediate black holes. The dense gas cocoon prevents stellar winds and at the same time ensures denser crowding among young stars. “The density of the stars is more than ten million cubic parsecs, which is more than enough to trigger a series of collisions,” astronomers say.

But more importantly, because of the tight crowding, the stars collide and merge so quickly that they barely have time to lose much of their mass to their powerful stellar winds. In the simulation, such collisions created at least one very massive giant star with a mass of at least 1,000 solar masses in each globular cluster. “This is much heavier than any star known today,” the team said. These short-lived extreme stars can go on to merge or collapse directly into a black hole – thus forming intermediate black holes.

And also in the Milky Way

According to astronomers' estimates, undiscovered intermediate black holes could also be hiding in ancient globular clusters of the Milky Way. “Such star clusters with an initial mass of more than 1 million solar masses could have produced black holes with masses exceeding 2,000 solar masses,” Fujii and her team wrote. Therefore, there is a good chance that at least some of the ancient, metal-poor globular star clusters in our region contain this rare intermediate form of black hole.

Now astronomers just need to find these hidden media, which is no easy task. The dense crowding in the center of the globular clusters also makes it difficult to detect such hidden black holes. (Science, 2024; doi: 10.1126/science.adi4211)

Source: University of Tokyo

May 31, 2024 – Nadia Podbrigar