Michiko Fujii of the University of Tokyo led a joint research project that demonstrated a possible formation mechanism of intermediate-mass black holes in globular clusters, star clusters that can contain tens of thousands or even millions of tightly packed stars. The research involved the first star-by-star massive cluster-formation simulations which revealed that dense molecular clouds, the “birthing nests” of star clusters, can give birth to very massive stars that evolve into intermediate-mass black holes. The findings were published in the journal Science, with the motivation for the research project being the previous observations of intermediate-mass black holes in massive star clusters.
In globular star clusters, the dense environment causes stars to collide and merge, leading to the growth of gravitational forces. Runway collisions in the central region of globular clusters can result in very massive stars with over 1000 solar masses, which could potentially evolve into intermediate-mass black holes. Previous simulations suggested that stellar winds blow away most of the mass of these stars, leaving them too small to evolve into black holes. To investigate if intermediate-mass black holes could “survive,” researchers needed to simulate a cluster while it was still forming, which presented challenges due to the simulation cost.
The researchers successfully performed numerical simulations of globular cluster formation for the first time, modeling individual stars with a realistic mass for each. By resolving individual stars, they could reconstruct the collisions of stars in a tightly packed environment and developed a novel simulation code that could integrate millions of stars with high accuracy. The simulation showed that runaway collisions led to the formation of very massive stars that evolved into intermediate-mass black holes, with the mass ratio between the cluster and the black hole matching observations that spurred the project.
The final goal of the research is to simulate entire galaxies by resolving individual stars, although simulating Milky Way-size galaxies is currently challenging due to the limitations of supercomputers. However, smaller galaxies such as dwarf galaxies could be simulated to further study the formation of globular clusters and intermediate-mass black holes. Additionally, the researchers aim to investigate the first clusters, star clusters that were formed in the early universe, as these are places where intermediate-mass black holes could potentially be born. The study opens up new avenues for understanding the formation mechanisms of black holes in globular clusters and their role in the evolution of galaxies.