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"Medium and powerful: medium-mass black holes can survive in globular clusters"

First-ever model simulations of individual stars in a star cluster in the process of formation suggest possible mechanisms for the formation of intermediate-mass black holes

A star cluster forms in a giant molecular cloud as reproduced by the simulation in the model. The picture is based on the model. The blue dots represent individual stars. Dark and light colors indicate the temperature of the gas (cold and hot). Visualization by Takaki Takeda (VASA ENTERTAINMENT INC.)
A star cluster forms in a giant molecular cloud as reproduced by the simulation in the model. The picture is based on the model. The blue dots represent individual stars. Dark and light colors indicate the temperature of the gas (cold and hot). Visualization by Takaki Takeda (VASA ENTERTAINMENT INC.)

Joint research led by Michiko Fujii of the University of Tokyo proposed a possible mechanism for the formation of intermediate-mass black holes in globular clusters, star clusters that can contain tens of thousands or even millions of densely packed stars. The first ever simulations of the formation of massive star clusters based on individual stars have revealed that molecular clouds of sufficient density are the birthplace of star clusters, and can give birth to very massive stars that evolve into medium-mass black holes. The findings were published in the journal Science.

"Previous observations suggested that some of the massive star clusters (globular clusters) contain an intermediate-mass black hole (IMBH)," explains Fujii of the reason for the research project. "A medium-mass black hole is a black hole with a mass of 100-10,000 solar masses. Until now, there has been no strong theoretical proof for the existence of intermediate-mass black holes of 1,000-10,000 solar masses compared to less massive (stellar mass) and more massive (supermassive) black holes."

The birth sites of the stars may evoke images of warmth and peace. Not so with stars. Globular star clusters form when the differences in density cause the stars to collide and merge. As the stars continue to merge and grow, the gravitational forces increase with them. The repeated interstellar collisions in the dense, central region of globular star clusters are called orbital collisions. These collisions can lead to the birth of very massive stars with more than 1,000 solar masses. These stars can potentially evolve into medium-mass black holes. However, previous simulations of already formed clusters suggested that stellar winds dissipate most of their mass, leaving them too small. To investigate whether intermediate-mass black holes can "survive," the researchers had to simulate a cluster while it was still forming.

"Simulations of the formation of star clusters have been challenging due to the cost of imaging," Fujii says. "For the first time, we were able to perform numerical simulations of the formation of a globular star cluster, using individual stars. By solving a model of individual stars with a realistic mass for each, we were able to reproduce the collisions of stars in a compressed environment. For these simulations, we developed an innovative simulation code, in which we could combine millions of stars with high precision."

In the simulation, the orbital collisions did lead to the formation of very massive stars that evolved into intermediate-mass black holes. The researchers also found that the ratio of the mass of the cluster to the intermediate-mass black hole matched the observations that motivated the project in the first place.

"Our ultimate goal is to simulate entire galaxies by solving a model of individual stars," notes Fuji for future research. "It is still difficult to simulate galaxies the size of the Milky Way by solving a model of individual stars using currently available supercomputers. However, we may be able to simulate smaller galaxies such as dwarf galaxies. We also want to target the first clusters, star clusters that formed in the early universe. The first clusters are also places where medium-mass black holes can be born."

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