A new study led by Manuel Arca Sedda, a researcher at the Gran Sasso Science Institute, published in the Monthly Notices of the Royal Astronomical Society (MNRAS), reveals the formation mechanism of mysterious intermediate-mass black holes (IMBHs). These objects, which range in mass from a few hundred to tens of thousands of solar masses, may represent a link between their smaller relatives, stellar black holes, and the supermassive holes at the centers of galaxies.

black hole spectrum

In fact, there are different types of black holes: although they are all very dense and even light cannot escape their gravity, the mass of these objects can vary within a wide range and distinguish the mechanisms of their formation. We can identify three macroscopic categories of astronomical interest: stellar, intermediate, and supermassive.

As the name suggests, the former is formed when a star with enough mass (that is, at least 20 times the mass of the sun) exhausts its fuel and collapses into its own interior under the influence of gravity: they represent the lightest type of black hole, and we already have a clear theoretical understanding of their formation process.

At the opposite extreme are massive supermassive black holes, millions or billions of times more massive than our stars. Every galaxy is believed to have a supermassive black hole at its center, and in 2019 we got the first direct image of one of them, thanks to the Horizon Event Telescope.

Despite this monumental achievement, the formation and accretion of these objects remains a fascinating mystery to modern astronomy, largely due to the lack of conclusive evidence for the existence of intermediate-mass black holes. This is the subject of Aka-Seda's study, the first of two other studies currently under review.

The image depicts a simulated star cluster calculated by the DRAGON-II simulation. The orange and yellow dots represent Sun-like stars, and the blue dots represent stars with masses between 20 and 300 times the mass of the Sun. The large white object in the center represents a star with a mass of about 350 solar masses, which will soon collapse to form an intermediate-mass black hole. Image source: M. Arca Sedda (GSSI)

The elusive intermediate-mass black hole

"Intermediate-mass black holes are difficult to observe," GSSI researchers explained. "Current observational limits do not allow us to make any judgments about IMBH groups with masses between 1,000 and 10,000 solar masses, and they also give scientists a headache in terms of possible formation mechanisms."

One of the goals of the research is to try to understand how these black holes form. "We have built new computer models that simulate the formation process of these mysterious objects. We found that such IMBHs can form in star clusters through a complex combination of three factors: mergers between stars much larger than the Sun, accretion of stellar material onto stellar black holes, and mergers between stellar black holes." Alca-Ceda explains: "The latter is a process that leads to the possibility of 'seeing' these phenomena through the detection of gravitational waves."

The study also hypothesized what happens after the intermediate black holes are born: they are ejected from their own clusters through complex gravitational interactions or due to a process called relativistic recoil, which prevents them from growing further.

An enlargement of a snapshot taken from a DRAGON-II simulation that models dense star clusters with up to 1 million stars. The orange and yellow dots represent Sun-like stars, and the blue dots represent stars with 20 to 300 times the mass of the Sun. The large white object in the center represents a star with a mass of about 350 solar masses, which will soon collapse to form an intermediate-mass black hole. Source: M. Arca Sedda (GSSI)

"Our model shows that although IMBH seeds form naturally from the interactions of energetic stars in star clusters, they are unlikely to weigh more than a few hundred solar masses unless the parent cluster is extremely dense or massive."

However, an important scientific mystery remains to be answered: whether the intermediate black hole represents a link between stellar and supermassive black holes. That's an open question, but this study provides some room for speculation.

"We need two elements to better clarify the issue, a process or processes capable of forming a black hole in the mass range of an IMBH, and the possibility of retaining such an IMBH in the host environment," explains Aka-Seda. "Our study puts strict constraints on the first of these, allowing us to "We have a clear idea of what processes may contribute to the formation of IMBHs. In the future, considering more massive star clusters containing more binaries (systems of two stars orbiting each other) may also be the key to obtaining the second element, but this will require a huge effort from a technical and computational perspective."

Compiled from /scitechdaily