New simulations suggest Webb’s Little Red Dots capture a rapid growth phase that could explain the early appearance of supermassive black holes.
Little Red Dots, the small, extremely red objects spotted by the James Webb Space Telescope in the early universe, may reveal how black holes grew so big, so quickly.
Astronomers have found supermassive black holes with masses millions or even billions of times that of the Sun less than 600 million years after the Big Bang. Explaining how they gained so much mass in that time has been difficult, and new simulations suggest the dots could represent the rapid growth needed to get them there.
Astronomers had expected Webb to help solve this puzzle by revealing fainter, more distant galaxies. Looking farther into space lets them look back in time, since light takes time to reach us. A galaxy whose light has traveled for 11 billion or 12 billion years appears as it was that long ago. Yet Webb’s deeper view of the early universe brought a puzzle of its own when it uncovered a large population of Little Red Dots whose nature was unclear.
Zooming in on black hole birth
To understand what could produce these objects, a team led by Sunmyon Chon at the Max Planck Institute for Astrophysics modeled how gas behaved in the early universe, following the connection between a galaxy’s surroundings and the individual clouds within that environment.
The researchers used ATERUI III, a supercomputer at the National Astronomical Observatory of Japan, to run the most detailed cosmological simulations to date. Its high-resolution computing capabilities made it possible to zoom in on those clouds while accounting for the conditions around them.
Visualization video of the simulation by ATERUI III showing the evolution of the Universe up to the appearance of supermassive black holes. Credit: Sunmyon Chon, Takaaki Takeda, 4D2U Project, NAOJ
Gas disks allow unusually rapid growth
Within the simulated clouds, radiation from neighboring galaxies changed the course of star formation. Intense far-ultraviolet radiation suppressed the formation of many smaller stars, allowing the gas to gather into a single supermassive star instead. When that star collapsed, it left behind a black hole seed, the starting point for a much larger black hole. Crucially, the seed formed within a dense disk of gas that trapped radiation. This surrounding material allowed the black hole to grow at rates dozens of times faster than would be possible under conditions in the modern universe.
The resulting black holes had properties that closely matched Webb’s Little Red Dots, supporting the idea that the telescope is seeing this early phase of rapid growth. The simulations also offered a reason such objects could be widespread. Both the formation of the seeds and their subsequent growth followed naturally from conditions in the early universe, without requiring exotic assumptions or rare coincidences to account for the many dots Webb has observed.
Reference: “Overmassive black holes and little red dots naturally form in simulations” by Sunmyon Chon, Shingo Hirano, Tomoaki Ishiyama, Seok-Jun Chang and Volker Springel, 16 September 2026, Nature.
DOI: 10.1038/s41586-026-10985-8
This work was supported by JSPS KAKENHI grant numbers JP21H01122 (T.I.), JP21K13960, JP21H01123 and JP26K00743 (S.H.).
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