Scientists unravel the mystery of mid-sized black holes in star clusters

Star cluster forming in a giant molecular cloud reproduced by the simulation. This image is based on the simulation. Blue dots represent individual stars. Dark and bright color indicate the gas temperatures (cold and hot). Credit: Michiko Fujii and Takaaki Takeda.

A team of researchers led by Michiko Fujii from the University of Tokyo has discovered a potential way that intermediate-mass black holes (IMBHs) could form in globular clusters.

Globular clusters are groups of stars that are tightly packed together, sometimes containing millions of stars.

The team’s findings, published in the journal Science, reveal how these black holes might come into existence through their detailed simulations.

The researchers used advanced computer simulations to study how star clusters form. They focused on dense molecular clouds, which are regions where stars are born.

These simulations, the first of their kind to look at clusters in such detail, showed that these dense clouds could produce very massive stars.

These stars, in turn, might evolve into intermediate-mass black holes.

Fujii explains that earlier observations hinted that some large star clusters might host these intermediate-mass black holes. These black holes have a mass between 100 and 10,000 times that of our Sun.

However, there hasn’t been strong theoretical evidence to support their existence, especially for those with a mass between 1,000 and 10,000 times the Sun’s mass.

This lack of evidence motivated the researchers to explore the idea further.

Despite their name, the birth of stars in these clusters is far from peaceful. The dense conditions in globular clusters cause stars to collide and merge frequently.

These repeated collisions, known as runaway collisions, can create extremely massive stars, each with over 1,000 times the mass of the Sun.

These massive stars could then evolve into IMBHs. But previous simulations suggested that stellar winds would strip away much of their mass, preventing them from becoming large enough. To see if IMBHs could indeed survive, the researchers needed to simulate the cluster formation process in detail.

Creating these simulations was challenging because they required a lot of computing power. Fujii’s team developed a new simulation code to model each star individually, which allowed them to accurately depict the star collisions in these dense environments.

Their simulations showed that runaway collisions did lead to the formation of very massive stars, which then evolved into intermediate-mass black holes. The mass ratios observed in their simulations matched those seen in real star clusters.

The ultimate goal for Fujii and her team is to simulate entire galaxies by modeling individual stars. Currently, it’s still difficult to simulate galaxies as large as the Milky Way with this level of detail, even with the best supercomputers available.

However, they believe it’s possible to simulate smaller galaxies, like dwarf galaxies. They also aim to study the first star clusters formed in the early universe, which could also be sites where IMBHs are born.

These findings bring us closer to understanding how intermediate-mass black holes form and highlight the importance of detailed simulations in uncovering the secrets of the universe.