Webb Telescope Finds Dense Star Clusters That May Have Birthed Black Hole Seeds

Dense star clusters in two very early galaxies seen by the James Webb Space Telescope may have been birthplaces for intermediate-mass black holes, according to a new study reported by Universe Today. Such black holes are thought to be the seeds that grew into the supermassive black holes of the modern Universe.

The results come from modeling, not from spotting the black holes themselves. The paper, published in Astronomy and Astrophysics, is led by Viola Bocchi of Sapienza University of Rome.

The numbers behind the finding

• 460 million years: The age of the Universe when the light of the Cosmic Gems galaxy, which is stretched by gravitational lensing, was emitted. Webb found five dense young star clusters there.
• 600 million years: The age of the Universe in Webb's image of the Firefly Sparkle galaxy, another lensed early galaxy. Webb found 10 young massive clusters there.
• 1600 to 2700 solar masses: The black hole seed masses the authors estimate for the Cosmic Gems clusters, the heaviest in the study.
• About 180 to 810 solar masses: The estimated range for Firefly Sparkle, at the lower boundary for intermediate-mass black holes.
• 100 to 100,000 solar masses: The rough mass range of intermediate-mass black holes, which sit between stellar-mass and supermassive black holes.

How the team got there

The clusters were too dense for standard simulations of every star, Universe Today reported. Instead, the researchers used a model based on runaway collisions between stars and on mass lost through stellar winds to estimate the black holes that would result. They then checked those results against direct simulations of compact star clusters.

Why the two galaxies differ comes down to compactness. The Cosmic Gems clusters are extremely compact, with half of their mass inside a radius of about 1 parsec, which helps runaway collisions build heavy seeds. The Firefly Sparkle clusters are larger, at about 4 to 6 parsecs, and generally less massive, so their cores are less able to produce heavy black holes.

Low metallicity, meaning few elements heavier than hydrogen and helium, also mattered. In clusters this young, few stars have lived and died to enrich the gas. That limits mass lost to stellar winds, so growing objects keep more of their mass.

Why it matters

Astrophysicists think supermassive black holes, some with billions of solar masses, grew from seeds that started as stellar-mass black holes. Intermediate-mass black holes are the missing middle step, and direct evidence for them is still thin. Universe Today noted the only direct evidence so far is a gravitational-wave signal from 2020, in which black holes of 85 and 66 solar masses merged into one of about 142 solar masses.

The new study suggests that dense clusters like these, which Webb has revealed in the early Universe, are natural places for such seeds to form. It also offers a rough estimate of how efficient the process could be, at the few percent level. Because the findings rest on models, they are a proposal for how the seeds arose, not a confirmed observation.

Around Prague, based on reporting by Universe Today