Researchers have discovered that elusive intermediate-mass black holes can form in dense star clusters containing 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 solar-mass black holes, which have masses between 10 and 100 solar masses, but are lighter than supermassive black holes, which have masses equal to millions or even 100 solar masses. billions from the suns.
These cosmic in-betweens, the first of which was discovered in 2012, have proven difficult to discover by astronomers. This star, called GCIRS 13E, 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. Path.
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 from subsequent mergers of increasingly massive black holes. But a star large enough to die and create a black hole with thousands of solar masses would be incredibly rare, and would struggle to maintain that mass when it “died.”
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To investigate the mystery of how these intermediate-mass black holes arise, a team of researchers conducted the first star-by-star simulation of massive clusters. This showed that a sufficiently dense molecular cloud “birth nest” of globular clusters could create stars massive enough to collapse and form an intermediate-mass black hole.
“Previous observations suggested that some large star clusters, globular clusters, host an intermediate-mass black hole,” team leader and University of Tokyo scientist Michiko Fujii said in a statement. said. “So far, no strong theoretical evidence has been found to demonstrate the existence of intermediate-mass black holes with 1,000 to 10,000 solar masses compared to less massive (stellar mass) and more massive (supermassive) black holes.”
A chaotic birthplace of black holes
The term “natal nest” may well conjure up images and feelings of warmth, comfort and peace, but it could not be less appropriate for star formation in globular clusters.
These densely packed star clusters live in chaos and mayhem, with density differences causing stars to collide and merge. This process results in stars accumulating mass, so that their gravitational pull increases, dragging more stars into their surroundings, thus leading to more and more mergers.
The uncontrolled collision and merger process that occurs at the heart of globular clusters can lead to the formation of stars with masses equivalent to about 1000 suns. This is enough mass to form an intermediate-mass black hole, but there is a catch.
Astrophysicists know that when stars collapse to form black holes, much of their mass is blown away by supernova explosions or stellar winds. Previous simulations of intermediate-mass black hole formation confirmed this and also suggested that even massive stars with 1000 solar masses would be too small to form an intermediate-mass black hole.
To discover whether a massive star could “survive” with enough mass to give birth to an intermediate-mass black hole, Fujii and his team simulated a globular cluster as it formed.
“For the first time, we have successfully performed numerical simulations of globular cluster formation by modeling individual stars,” said Fujii. said. “By resolving individual stars with a realistic mass for each, we were able to recreate collisions of stars in a tight environment. For these simulations, we developed a new simulation code with which we could 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 retain enough mass to collapse and give rise to an intermediate-mass black hole.
The team also found that the simulation predicted the mass ratio between the intermediate-mass black hole and the globular cluster in which it formed. It turned out that this ratio coincides with real astronomical observations.
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“Our ultimate goal is to simulate entire galaxies by resolving individual stars,” Fujii said. he explained. “It is still difficult to simulate Milky Way-sized galaxies by resolving individual stars using currently available supercomputers. But it may be possible to simulate smaller galaxies, such as dwarf galaxies.”
Fujii and his team also plan to target star clusters that formed in the early universe. “Protoclusters are also places where intermediate-mass black holes can be born,” he said.
The team’s research was published Thursday (May 30) in the journal Science.