Intermediate-mass black holes (IMBHs) are a cosmic enigma, their very existence and mechanisms of formation shrouded in mystery
A recent study led by Manuel Arca Seda of the Gran Sasso Institute of Science and published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS), sheds light on the mechanisms leading to the formation of intermediate-mass black holes (IMBHs). These are objects with masses of several hundred to tens of thousands of solar masses, which can form the link between their smaller relatives, stellar black holes, and the supermassive giants that inhabit the centers of galaxies.
The spectrum of black holes
There are different types of black holes: although they all have such high densities that even light cannot escape their gravity, the mass of these celestial bodies can vary over a very wide range and they differ in the mechanism of their formation. We can identify three main categories of black holes: stellar, intermediate and supermassive.
Black holes in the first group, as the name suggests, are formed when a sufficiently massive star (that is, at least twenty times more massive than our Sun) exhausts its fuel and collapses in on itself: they represent the lowest-mass type of black hole, and we have an understanding theoretically clear about the process leading to their formation.
At the opposite end are enormous supermassive black holes, with masses millions or billions of times greater than our star. Each galaxy is believed to host one supermassive black hole at its center and in 2019, thanks to the Event Horizon Telescope, it was possible to obtain the first direct image of one.
Regarding the group of medium-mass black holes, the formation and accumulation of these objects are still a fascinating mystery to modern astronomy, mainly due to the lack of unequivocal evidence for the mechanism of formation and existence of these black holes.
The image depicts a simulation of a star cluster as produced by Dragon-II simulations. Orange and yellow dots represent Sun-like stars, while the blue dots indicate 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 and form a medium-mass black hole. Credit: © M. Arca Sedda (GSSI)
The elusive medium-mass black holes
"Intermediate mass black holes are difficult to observe," explains the researcher from GSSI, "Current observational limitations do not allow us to say anything about the population of IMBHs with masses between 1,000 and 10,000 solar masses, and they also represent a headache for scientists in terms of the possible leading mechanisms to their formation".
One of the goals of the research was actually to try to understand how these black holes are formed. We ran new computer models that can simulate the formation of these mysterious objects, and found that such IMBHs can form in star clusters through a complex combination of three factors: mergers between stars much larger than our Sun, accretion of stellar material onto stellar black holes, and finally, mergers between holes Starry blacks. The latter is a process that makes it possible to 'see' these phenomena through the detection of gravitational waves," explains Arca Seda.
The study also speculates what happens after medium-sized black holes are born: they are ejected from the clusters in which they were formed through complex gravitational interactions or as a result of a process known as relativistic recoil, thus preventing their further mass accretion.
"Our models show that although IMBHs form naturally from energetic interstellar interactions in star clusters, they are unlikely to be heavier than a few hundred solar masses, unless the parent cluster is very dense or massive," says a GSSI researcher.
However, an important scientific mystery remains unsolved: whether intermediate black holes are the link between stellar and supermassive black holes. That's an open question, but the research leaves room for some speculation.
"We need two components in order to reach a better understanding," explains Arka Seda, "one or more processes capable of creating black holes in the mass range of IMBHs, and the possibility of keeping such IMBHs in the host environment. Our study places strict constraints on the first component, giving us an overview It is clear which processes may contribute to the formation of IMBHs. Considering more massive clusters containing more binary star systems (systems consisting of two orbiting stars each other) in the future may be the key to achieving the second component as well. But it will require tremendous efforts technologically."
for the scientific article https://academic.oup.com/mnras/article/526/1/429/7281010
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