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The Rigakubu News

Disclaimer: machine translated by DeepL which may contain errors.

Intermediate-mass black holes exist in globular clusters ⁉︎

Michiko Fujii, Associate Professor, Department of Astronomy

 

While globular clusters have been known for a long time, their formation process is still shrouded in mystery. State-of-the-art numerical simulations have enabled us to reproduce the formation of globular clusters from molecular clouds, which are the parent bodies of stars. In the forming globular clusters, stars merge with each other to form supermassive stars, which are 10,000 times more massive than the Sun. Such stars are thought to eventually become intermediate-mass black holes, and this simulation strongly supports the observation that intermediate-mass black holes exist at the centers of globular clusters.

The existence of black holes in the universe has been confirmed by various observations. However, some of these observations have been highly certain, while others have been less certain. There is observational evidence of stellar black holes (less than 100 times the mass of the sun) and giant black holes (more than 100,000 times the mass of the sun at the center of a galaxy). Among intermediate-mass black holes, there is no unanimous observational evidence for intermediate-mass black holes with masses of several hundred to ten thousand times the mass of the Sun, and their existence and formation process have been debated for many years. One of the candidates for the location of such intermediate-mass black holes is a globular cluster. A globular cluster is a spherical object consisting of millions of stars gravitationally bound to each other, and several observations have suggested the existence of an intermediate-mass black hole at its center.

A simulation of a globular cluster in the process of formation
Each of the blue-white dots in the lower left represents a star in the cluster, and the “mist” around them represents interstellar gas. The color indicates temperature, with the dark areas representing low-temperature interstellar gas (molecular clouds) and the bright areas representing high-temperature interstellar gas. Credit: Michiko Fujii (University of Tokyo), Takaaki Takeda (Vaisa Entertainment Co., Ltd.)

Collisional mergers between objects are one of the leading theories for the formation of intermediate-mass black holes in globular clusters. Numerous numerical simulations have investigated the formation process of intermediate-mass black holes in globular clusters. However, there are two possible explanations for the formation of intermediate-mass black holes: either the black holes merge repeatedly in the cluster but fly out of the cluster due to anisotropic gravitational wave emission before the mass exceeds 500 times the solar mass, or the stars merge but lose their mass due to strong stellar wind (a flow of gas blown out from the star, stronger for more massive stars) and become The result is a stellar-mass black hole, and the formation of an intermediate-mass black hole several thousand times the mass of the Sun is ruled out. While these simulations were performed on already formed star clusters, in this study, for the first time, we simulated the process of star formation in a molecular cloud, which is the parent body of stars, including collisions and mergers among stars. In the simulation, stars merged one after another in the cluster in the process of formation, eventually forming a supermassive star with a mass about 10,000 times that of the Sun. According to calculations based on the theory of stellar evolution, such a supermassive star would eventually become an intermediate-mass black hole with a mass three to four thousand times that of the Sun. The relationship between the cluster mass and the black hole mass formed in the simulation is consistent with the relationship between the mass of the globular cluster and the mass of the black hole estimated from observations, and the simulation of globular cluster formation theoretically suggests the existence of an intermediate-mass black hole in a globular cluster The simulation of globular cluster formation theoretically suggests the existence of intermediate mass black holes in globular clusters. Further detailed observations of globular clusters will confirm whether intermediate-mass black holes exist in the center of globular clusters.

The results of this study were published in M. Fujii et al. Science, 384, 1488 (2024).

 

(Press release, May 31, 2024)

Published in The Rigakubu News, September 2024

 

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