Joint research led by Michiko Fujii of the University of Tokyo has unveiled a possible formation mechanism for intermediate-mass black holes (IMBHs) in globular clusters. These clusters, which can contain tens of thousands to millions of tightly packed stars, have long been suspected to host IMBHs, but direct theoretical evidence has been lacking until now. The groundbreaking findings were published in the journal Science.
The research involved the first-ever star-by-star massive cluster-formation simulations, revealing that sufficiently dense molecular clouds, often referred to as the “birthing nests” of star clusters, can produce very massive stars. These stars have the potential to evolve into IMBHs, bridging the gap between stellar-mass black holes and supermassive black holes.
“Previous observations have suggested that some massive star clusters (globular clusters) host an intermediate-mass black hole,” said Fujii, explaining the impetus behind the study. “An IMBH is a black hole with a mass ranging from 100 to 10,000 solar masses. Despite observational hints, there has been no strong theoretical evidence supporting the existence of IMBHs with masses between 1,000 and 10,000 solar masses.”
Globular star clusters are formed in chaotic environments. The dense conditions within these clusters lead to frequent stellar collisions and mergers. As stars continue to collide and grow, their increasing gravitational forces foster even more collisions. This cycle, known as runaway collisions, can produce very massive stars exceeding 1,000 solar masses, which could potentially evolve into IMBHs. However, previous simulations of already-formed clusters indicated that stellar winds would strip these massive stars of much of their mass, preventing the formation of IMBHs. To test the survivability of IMBHs, researchers needed to simulate cluster formation from the beginning.
“Star cluster formation simulations were challenging because of the simulation cost,” Fujii noted. “For the first time, we successfully performed numerical simulations of globular cluster formation by modeling individual stars. By resolving individual stars with realistic masses, we could accurately reconstruct the collisions in a tightly packed environment. We developed a novel simulation code that allowed us to integrate millions of stars with high precision.”
The simulations showed that runaway collisions did indeed lead to the formation of very massive stars, which then evolved into intermediate-mass black holes. Furthermore, the mass ratio between the cluster and the IMBH observed in the simulations matched that seen in actual observations, providing strong theoretical backing for the existence of IMBHs.
“Our final goal is to simulate entire galaxies by resolving individual stars,” Fujii said. “Currently, simulating Milky Way–size galaxies with individual stars is beyond the capabilities of available supercomputers. However, it is feasible to simulate smaller galaxies, such as dwarf galaxies. We also aim to target the first star clusters formed in the early universe, which are also potential birthplaces for IMBHs.”
This research marks a significant advance in our understanding of black hole formation and the dynamics of star clusters. By providing a clearer picture of how IMBHs can form and survive within globular clusters, these findings open new avenues for exploring the role of black holes in the evolution of galaxies and the universe.
Source: University of Tokyo