Researchers have discovered that 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 heavier than black holes with masses of about 10 to 100 solar masses, yet lighter than supermassive black holes, which have masses equal to millions or even billions of sunbathing.
These cosmic intermediate bodies have proven elusive for astronomers to discover; the first copy was found in 2012. Called GCIRS 13E, it has a mass 1,300 times that of the Sun and is located 26,000 light-years away, toward the galactic center of the Milky Way. Way.
One of the mysteries surrounding intermediate-mass black holes concerns their formation. Stellar-mass black holes form when massive stars collapse, and supermassive black holes grow from subsequent mergers of increasingly larger black holes. Still, a star big enough to die and create a black hole with thousands of solar masses would have to be incredibly rare and struggle to retain that mass when it “dies.”
Related: Again, Einstein! Scientists discover where matter ‘falls’ into black holes
To investigate the mystery of how these intermediate-mass black holes form, a team of researchers has performed the first-ever star-by-star simulation of massive clusters. This showed that a ‘birth nest’ of globular clusters from a dense enough molecular cloud could create stars large enough to collapse and produce an intermediate-mass black hole.
“Previous observations have suggested that some massive star clusters and globular clusters harbor an intermediate-mass black hole,” said team leader and University of Tokyo. scientist Michiko Fujii said in a statement. ‘Until now, there has been no strong theoretical evidence demonstrating 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.’
A chaotic birthplace for black holes
The term ‘birth nest’ may conjure up images and feelings of warmth, comfort and tranquility, but this is no less applicable to star formation in globular clusters.
These densely packed conglomerates of stars live in chaos and unrest, with differences in density causing stars to collide and merge. That process causes stars to pile up in mass, increasing their gravitational influences, dragging more stars into their environment and causing more and more mergers.
The runaway collision and merger process that takes place in the hearts of globular clusters can lead to the creation of stars with a mass of about 1,000 suns. That’s enough mass to create an intermediate-mass black hole, but there’s a hurdle.
Astrophysicists know that when stars collapse to create black holes, much of their mass is blown away by supernova explosions or by stellar winds. Previous simulations of the creation of intermediate-mass black holes have confirmed this, further suggesting that even massive stars of 1,000 solar masses would eventually become too small to create an intermediate-mass black hole.
To discover whether a massive star could “survive” with enough mass to give rise to an intermediate-mass black hole, Fujii and his team simulated a globular cluster as it formed.
“We have successfully performed numerical simulations of globular cluster formation for the first time, modeling individual stars,” Fujii said. ‘By resolving individual stars with a realistic mass for each, we can reconstruct the collisions of stars in a close-packed environment. For these simulations we have developed a new simulation code in which we can integrate millions of stars with high accuracy.’
In the simulated globular cluster, runaway collisions and mergers led to the formation of extremely massive stars that could hold enough mass to collapse and create an intermediate-mass black hole.
The team also found that the simulation predicted a mass ratio between the intermediate-mass black hole and the globular cluster in which it forms. That ratio, it turned out, is consistent with actual astronomical observations.
“Our ultimate goal is to simulate entire galaxies by resolving individual stars,” Fujii explains. “It is still difficult to simulate galaxies the size of the Milky Way by resolving individual stars using currently available supercomputers. However, it would be possible to simulate smaller galaxies, such as dwarf galaxies.”
Fujii and her team also plan to focus on star clusters that formed in the early universe. “The first clusters are also places where intermediate-mass black holes can form,” she says.
The team’s research was published Thursday (May 30) in the journal Science.