JWST observations of distant galaxies have revealed incredibly dense and bright clusters of stars in the early universe. New simulations show how these clusters may have formed through galactic disk fragmentation.
From the Early Universe to the Present Day
While JWST observations were expected to shed light on the earliest stages of galaxy evolution, certain discoveries raised more questions than answers. How structures like galactic disks, supermassive black holes, and massive star clusters developed so rapidly is an open question.
The gas density (left) and stellar density (right) for the main galaxy, labeled “0,” and its two galactic companions. Click to enlarge. [Adapted from Mayer et al. 2025]
In the wake of the galaxy collision, the resulting galaxy forms a dense, gas-rich disk. Because of the high density of this gas disk, fragmentation of the disk into massive star clusters occurs quickly, on timescales of a million years. In less extreme environments, the rate of star formation is moderated by feedback — for example, from supernova explosions that inject energy and momentum into star-forming clouds, heating and disrupting the clouds and slowing the rate of star formation. Here, however, fragmentation happens too fast for supernova feedback to stop it, resulting in massive, dense clumps of gas that collapse further to form star clusters.
The gravitationally lensed Cosmic Gems arc, imaged by JWST at a redshift of z = 10.2 (about 500 million years after the Big Bang) bears considerable similarity to the smallest of the three galaxies in this simulation. [ESA/Webb, NASA & CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration; CC BY 4.0]
Cluster Assemblage
The central galaxy and its two smaller companions all formed star clusters through this method. The mass of the clusters appears to scale with the mass of the galaxy, with the largest galaxy forming the largest number of clusters and the most massive clusters. In total, roughly 20–30% of the total stellar mass of each galaxy forms through fragmentation of the disk, creating clusters with masses of 105–108 solar masses. The large fraction of galactic stellar mass belonging to massive star clusters, as well as the rapid time frame of their formation — the simulation spans just 6 million years — are both consistent with JWST’s observations of star clusters in the early universe.
These star clusters may provide a clue to another early universe mystery: the development of supermassive black holes. The extreme density of the simulated star clusters provides a natural avenue for the formation of intermediate-mass black holes with masses up to 105 solar masses. These black holes could sink to the center of the galaxy and coalesce to form a single supermassive black hole with a mass of 10 million solar masses — comparable in mass to many of the black holes seen with JWST.
Intriguingly, in the smallest galaxy explored in the team’s simulation, the black hole’s mass would be only a factor of a few less massive than the total mass of stars in the galaxy, potentially also explaining why some galaxies appear to have over-massive black holes.
Citation
“In Situ Formation of Star Clusters at z > 7 via Galactic Disk Fragmentation: Shedding Light on Ultracompact Clusters and Overmassive Black Holes Seen by JWST,” Lucio Mayer et al 2025 ApJL 981 L28. doi:10.3847/2041-8213/adadfe