The researchers found that the elusive intermediate-mass black holes can form in dense star clusters containing anywhere from tens of thousands to millions of tightly packed stars called “globular clusters.”
An intermediate mass black hole has a mass between 100 and 10,000 suns. They are more massive than solar-mass black holes, which have a mass range between 10 and 100 solar masses, but lighter than supermassive black holes, which have masses equivalent to millions or even billions the sun.
These cosmic intermediates have proved elusive to astronomers since the first example was found in 2012. Designated GCIRS 13E, it has a mass 1,300 times that of the Sun and is located 26,000 light-years away, towards the Milky Way’s galactic center. Way.
One of the mysteries surrounding intermediate-mass black holes concerns their formation. Stellar-mass black holes are born when massive stars collapse, and supermassive black holes grow by the subsequent merger of larger and larger black holes. Still, a star massive enough to die to form a black hole with thousands of solar masses should be incredibly rare and try to retain that mass when it “dies”.
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To explore the mystery of how these intermediate-mass black holes form, a team of researchers conducted the first-ever simulation of massive star-by-star clusters. This showed that the dense enough molecular cloud “birth nest” of globular clusters could produce stars massive enough to collapse to form an intermediate-mass black hole.
“Previous observations suggested that some massive star clusters, globular clusters, host an intermediate-mass black hole,” team leader and the University of Tokyo scientist Michiko Fujii said in a statement. “There is no strong theoretical evidence yet for the existence of an intermediate-mass black hole with a mass of 1,000 to 10,000 solar masses compared to less massive (stellar mass) and more massive (supermassive) ones.”
The chaotic birthplace of black holes
The term “natal nest” may well conjure up images and feelings of warmth, comfort and peace, but it couldn’t be less appropriate for star formation in globular clusters.
These densely packed conglomerates of stars live in chaos and confusion, with differences in density causing stars to collide and merge. This process causes stars to accumulate mass, increasing their gravitational influence, drawing more stars into their vicinity, and thus more and more mergers occur.
The uncontrolled collision and merger process that occurs at the heart of globular clusters can give rise to stars with masses equivalent to about 1000 suns. That’s enough mass to create a medium-mass black hole, but there’s a catch.
Astrophysicists know that when stars collapse to form black holes, much of their mass is blown away in supernova explosions or stellar winds. Previous simulations of the formation of intermediate-mass black holes have confirmed this, further suggesting that even massive stars of 1,000 solar masses would be too small to form an intermediate-mass black hole.
To see if a massive star could “survive” with enough mass to give birth to an intermediate-mass black hole, Fujii and team simulated a globular cluster as it formed.
“For the first time, we have successfully performed numerical simulations of the formation of globular clusters, modeling individual stars,” Fujii said. “By splitting individual stars with realistic masses each, we could reconstruct stellar collisions in a tightly packed environment. For these simulations, we developed a new simulation code that could integrate millions of stars with high precision.”
In the simulated globular cluster, runaway collisions and mergers led to the formation of extremely massive stars that could retain enough mass to collapse and give birth to intermediate-mass black holes.
The team also found that the simulation predicted the mass ratio between the intermediate-mass black hole and the globular cluster in which it forms. This ratio, as it turned out, corresponds to real astronomical observations.
“Our ultimate goal is to simulate entire galaxies by resolving individual stars,” Fujii explained. “It is still difficult to simulate galaxies the size of the Milky Way using the resolution of individual stars using currently available supercomputers. However, it would be possible to simulate smaller galaxies such as dwarf galaxies.”
Fujii and her team also intend to focus on star clusters formed in the early universe. “Primitive clusters are also places where intermediate-mass black holes can be born,” she said.
The team’s research was published Thursday (May 30) in the journal Science.