Space, this is how intermediate black holes are created: the Italian study

A recent study, led by Manuel Arca Sedda, researcher at the Gran Sasso Science Institute, and published in the prestigious journal “Monthly Notices of the Royal Astronomical Society” (Mnras), sheds light on the mechanisms that lead to the formation of the mysterious intermediate black holes ( Imbhs)

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These are objects with masses between a few hundred and tens of thousands of solar masses, which could represent the link between their smaller relatives, stellar black holes, and the supermassive giants that populate the centers of galaxies

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In fact, there are different types of black holes: although they share such high densities that not even light can escape their gravitational attraction, the mass of these celestial bodies can vary over a very wide range and discriminate their formation mechanism

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Three macro-categories of astronomical interest can be identified: stellar, intermediate, and supermassive. The former, as the name suggests, are formed when a star of sufficiently large mass, i.e. at least twenty times more massive than our sun, runs out of fuel and succumbs to the force of gravity by collapsing on itself: they represent the lightest type of hole black and there is a very clear theoretical framework on the process that leads to their formation (in the photo, the gravitational force created by a black hole)

At the other extreme there are the immense supermassive black holes, with masses millions or billions of times greater than our star (in the photo, the formation of a black hole)

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It is believed that every galaxy hosts one at its center and, in 2019, thanks to the Event Horizon Telescope, it was possible to obtain a first direct image of the one at the center of ours, the Milky Way (its characteristics in the photo)

Despite this result, the formation and accretion of these objects still represents a fascinating mystery for modern astronomy, especially due to the lack of definitive evidence to support the very existence of intermediate mass black holes.

And this is precisely the theme of Arca Sedda’s study, the first of two others currently under review. “Black holes of intermediate mass are difficult to observe – explains the GSSI researcher – just think that the current observational limits do not allow us to say anything about the Imbh population with masses between 1,000 and 10,000 solar masses and represent a headache for scientists also with regard to the possible mechanisms that lead to its formation”

So one of the objectives of the research was to try to understand how these are formed. “We have carried out new computer models capable of simulating the formation of these mysterious objects, and we have found that such Imbhs can form into star clusters through a complex combination of three factors: mergers between stars much larger than our sun, the accretion of stellar material onto stellar black holes and, finally, mergers between stellar black holes

The latter is a process that results in the possibility of ‘seeing’ these phenomena through the detection of gravitational waves”, explains Arca Sedda. The study also hypothesizes what happens after the birth of intermediate black holes: they are thrown away by their own clusters through complex gravitational interactions or due to a process known as relativistic recoil, all of which prevents their growth

“Our models show that although Imbh seeds form naturally from energetic stellar interactions in star clusters, they are unlikely to become heavier than a few hundred solar masses unless the parent cluster is extremely dense or massive.” . However, an important scientific question remains to be clarified, namely whether intermediate black holes are the link between stellar black holes and supermassives. This is an open question, but the study gives room for some hypotheses

“To answer we need 2 ingredients – says Arca Sedda – one or more processes capable of forming black holes well within the mass range of the Imbh, and the possibility of retaining such Imbh in the host environment. The study places stringent limits on the first ingredient, providing an overview of which processes can contribute to the formation of Imbhs. Considering more massive clusters containing a greater number of binaries in the future could be the key to obtaining the second ingredient. But this will require further enormous efforts”

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