Modern supermassive black holes, whose mass reaches billions of solar masses, according to astrophysicists, grew through a long process of successive mergers. But where did their "seeds" come from? An answer may be provided by a new study published in the journal Astronomy and Astrophysics under the leadership of Viola Bocca of Sapienza University of Rome.
The subject is intermediate-mass black holes - objects with masses of roughly 100 to 100,000 solar masses. Direct observations of such black holes are still few, but they are gradually accumulating. For example, in 2020 gravitational waves - oscillations of space-time arising from the merger of massive objects - showed the merger of two black holes with masses of 85 and 66 solar masses into an object of about 142 solar masses. And in 2024, the motion of fast stars at the center of the globular cluster Omega Centauri indicated the presence there of a black hole with a mass of about 8,200 solar masses.
The authors of the new work drew attention to two ancient objects that the James Webb Space Telescope (JWST) observed thanks to gravitational lensing - an effect in which the gravity of a large foreground galaxy bends and magnifies the light of a distant object. These are the Cosmic Gems arc, whose light traveled for about 460 million years after the Big Bang, and the galaxy Firefly Sparkle, visible roughly 600 million years after it.
In the Cosmic Gems arc the telescope found five dense young massive clusters, and in Firefly Sparkle ten. The scientists built a model describing so-called runaway stellar collisions, in which stars in a dense cluster collide and merge, forming an increasingly massive object, and took into account mass loss due to stellar winds. The model was tested against direct numerical simulations of compact clusters.
The calculations showed that black holes with masses of 1,600 to 2,700 solar masses could have been born in the Cosmic Gems clusters. A key role was played by low metallicity - the content of elements heavier than helium: it limits mass loss under the action of stellar winds and allows the "embryo" to grow. In Firefly Sparkle the clusters are larger in radius and less dense, so lower masses are expected there - roughly 180 to 810 solar masses.
The authors emphasize: this is not an observation of intermediate-mass black holes, but a simulation of the environment where they could form. Many properties of the clusters, including rotation and internal structure, are unknown and could change the estimates. Nevertheless, the researchers conclude that dense, metal-poor clusters could have been "factories" of heavy seeds from which supermassive black holes grew in the early Universe.