Two research teams, working independently, have converged on the same surprising answer to one of astronomy's biggest puzzles: where the seeds of supermassive black holes came from.

According to phys.org, a study led by Viola Bocchi of Sapienza University of Rome, published in Astronomy & Astrophysics, applied collision-based models to dense young massive clusters that JWST discovered in the gravitationally lensed galaxies Cosmic Gems (z ~ 10.2) and Firefly Sparkle (z ~ 8.3).

Meanwhile, phys.org's Universe Today reports a companion line of modeling, covered by space.com via ua.news, showed that repeated stellar mergers in dense early-universe clusters could build a single extremely massive star that collapses into a massive black hole.

The new work points to intermediate-mass black holes (IMBHs) — the poorly observed range between ~100 and 100,000 solar masses — as the heavy seeds that bridge the gap.

Direct evidence for IMBHs is thin: phys.org notes a 2024 Omega Centauri study inferring an ~8,200-solar-mass IMBH from fast-moving stars, and a 2020 gravitational-wave detection of a ~142-solar-mass merger remnant.

The modeling, per space.com as reported by ua.news, suggests slow collisions in hyper-dense early clusters let stars merge repeatedly into one object with the mass of the whole cluster.

The JWST found five young massive clusters in the Cosmic Gems arc, whose light left when the universe was about 460 million years old, and ten in Firefly Sparkle, seen roughly 600 million years after the Big Bang.

phys.org reports the Bocchi team's estimates put typical IMBH masses at ~10² to 4 × 10³ solar masses, with the extremely compact Cosmic Gems clusters (half-mass radius ~1 pc) yielding the heaviest seeds at 1,600–2,700 solar masses.

Low metallicity was critical — it limits mass loss from stellar winds, letting the seeds keep growing. In Firefly Sparkle's looser clusters (Rh ~ 4–6 pc), the estimated black holes reached only ~180–810 solar masses, the lower boundary of the IMBH range.

The results may also explain JWST's mysterious 'little red dots' — compact red objects in the early universe whose spectra are often interpreted as powered by supermassive black holes. Both teams suggest dense massive star clusters could be the massive seed beds from which those SMBHs grew.

According to phys.org, the authors conclude that these dense, metal-poor clusters are 'viable factories' for heavy seeds, while caveats remain: cluster rotation, internal structure, and the initial mass function are unknown and could shift the outcomes.

ua.news, citing space.com, notes little red dots appear to vanish by the time the universe was ~2 billion years old, when expansion and cooling ended the chaotic collision conditions — no IMBHs were actually observed, so follow-up work will test whether the factories are real.