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How to make a midsize black hole

How to make a midsize black hole

20 June 2024

As detailed simulations of star formation show, runaway collisions in a giant molecular cloud may produce very massive stars that are precursors to intermediate-mass black holes.

Illustration of densely packed stars in a molecular cloud.
A simulation of a star cluster forming in a giant molecular cloud, as visualized by this artistic rendering, indicates that runaway collisions can create the very massive stars needed to produce intermediate-mass black holes. Blue dots represent individual stars. Credit: Michiko Fujii and Takaaki Takeda, 2024

Found at the center of galaxies, supermassive black holes have the mass of more than 100 000 suns. Stellar-mass black holes, widely observed in binary systems in which a black hole siphons material from its partner star, are typically less than 50 solar masses. But observations of black holes with masses somewhere in between have been relatively sparse and sometimes contentious, and the ways that those intermediate-mass black holes might form remain a subject of debate. New star-by-star simulations by Michiko Fujii (University of Tokyo) and colleagues lend support to one possibility: The birth of stars in dense clusters can lead to the formation of those mysterious midsize black holes.

Dense groups of stars known as globular clusters, found in the galactic halo of the Milky Way, are considered one of the likely hosts of intermediate-mass black holes. Fujii and colleagues have developed a detailed code that can contain hundreds of thousands to millions of individually resolved stars. The code has been used to simulate the birth of the Orion Nebula and now globular clusters from their inception.

The simulations begin with a giant molecular gas cloud that contains 105–106 solar masses worth of matter. The high density of gases nucleates stars that proceed to grow, interact, and merge amid the cloud. Other models of globular clusters have focused on either star formation or stellar collisions. Fujii and colleagues found that combining the two creates the conditions for the runaway collisions needed to produce a very massive star.

Much larger than any stars ever observed by humans, very massive stars (more than 103 solar masses) are predicted to form from multiple mergers of large stars before collapsing into black holes. But as stars grow larger, they generate stronger stellar winds that cause them to lose mass. The competition between mass loss and mass accumulation can thus limit star size. The new study shows that the density of the gas cloud around the stars adds gravitational potential that keeps the star cluster compact, so that more stars merge with each other and mass accumulates faster than it is lost. Eventually a star will form that is large enough to collapse into a midsize black hole.

Among the many questions researchers have about intermediate-mass black holes is whether they aid in the growth of their supermassive counterparts. Although findings show that some of the largest observed supermassive black holes formed when the universe was less than a billion years old, it’s still unclear how they grew so quickly.

As the hunt for more observations of intermediate-mass black holes continues, Fujii and colleagues’ simulations confirm that globular clusters are a good target in the search. Fujii plans to continue expanding the size of her star-by-star simulations, with the goal of modeling dwarf galaxies and, eventually, galaxies the size of the Milky Way. (M. S. Fujii et al., Science, 2024, doi:10.1126/science.adi4211.)

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