JWST's Dense Baby Clusters May Be Where Heavy Black Hole Seeds Were Born

Supermassive black holes have a timing problem. Telescopes keep finding them fully grown at epochs when the universe seems too young to have built them, at least if they started as the remnant of a single ordinary dead star. One proposed fix is that some black holes got a head start. In that picture they were born as "heavy seeds" of hundreds or thousands of solar masses rather than the tens of solar masses typical of a single collapsed star. That idea pushes the question back a step, though. Where in the infant universe could you actually make such a thing?

A study accepted for publication in Astronomy and Astrophysics gives a concrete answer, and it points at objects the James Webb Space Telescope has already imaged. The paper is led by Viola Bocchi of Sapienza University of Rome, with co-authors Liempi and Schleicher. It looks at the compact young star clusters JWST resolved in two gravitationally lensed systems, the Cosmic Gems arc and the Firefly Sparkle. The team concludes that these clusters could plausibly form intermediate-mass black holes (IMBHs) of roughly 100 to 4,000 solar masses through runaway collisions between stars.

Two snapshots of the young universe

Both targets are seen magnified by foreground gravitational lenses, and that magnification is the only reason individual clusters can be picked out at such distances. The Cosmic Gems arc sits at a redshift of about 10.2. As Universe Today reports, that means we see it roughly 460 million years after the Big Bang, and it holds five young massive clusters. The Firefly Sparkle, at a redshift of about 8.3, is seen around 600 million years after the Big Bang and contains 10 clusters.

The clusters in these systems are heavy and tightly packed. According to the arXiv paper, they range from about 100,000 to 10 million solar masses, with half-mass radii from roughly 0.4 to 15 parsecs. Density is what matters here. When that much stellar mass is crammed into a region a parsec or so across, stars stop being isolated objects on separate orbits and start running into each other.

How a cluster builds a black hole

The mechanism the authors model is a runaway collision process. In a dense cluster, the most massive stars sink toward the center through gravitational interactions with lighter neighbors. As the core fills up, collisions become likely. A star that has merged once is bigger and heavier, so it is an easier target for the next collision. That feedback loop can produce a single very massive star, which may eventually collapse into a black hole far heavier than a normal stellar remnant.

To estimate how far that process goes, the team used a Fokker-Planck model, a statistical treatment of how a cluster's stars evolve over time, together with an analytic framework for runaway collisions. They checked that framework against N-body simulations, which follow individual stellar orbits directly. The analytic approach makes it cheap to sweep through the range of cluster masses and sizes JWST actually measured, instead of running a full simulation for each object.

The headline numbers: typical IMBH masses come out between about 100 and about 4,000 solar masses, and the formation efficiency is a few percent. In other words, a few percent of a cluster's stellar mass ends up in the central black hole. Results differ a lot between the two systems:

Cosmic Gems: the clusters have half-mass radii around 1 parsec and yield seeds of roughly 1,600 to 2,700 solar masses.
Firefly Sparkle: the clusters are more spread out (radii of about 4 to 6 parsecs) and range from 100,000 to 1 million solar masses. They produce smaller IMBHs, about 180 to 810 solar masses.

The difference between the two follows the physics. More compact clusters have higher central densities and more collisions, so they grow bigger black holes.

The metallicity catch

There is a condition attached. The paper finds that low metallicity is critical, with metallicity below roughly 2 percent of the Sun's (Z ≲ 0.02 Z_☉). Stellar winds strip mass from massive stars, and low metallicity is what lets the growing star retain its mass against those winds. Universe Today adds that young clusters are metal-poor because few earlier generations of stars have lived and died in them to enrich the gas. That makes the early universe, before many generations of stars had enriched the gas with heavier elements, an especially good setting for this route.

A second, independent line of argument

The Sapienza-led paper is not the only recent work heading this way. Space.com reports on simulations by a UCLA team including Claire Williams and Smadar Naoz, which reach a broadly similar picture. In their models, dense early star clusters allow slower collisions that lead to repeated mergers, building single over-massive stars that can collapse into larger-than-usual black holes, which could be seeds of the first supermassive black holes. The UCLA researchers suggest that the compact, puzzling objects JWST has found and nicknamed "Little Red Dots" may be clusters of this kind.

The two studies differ in method and emphasis. One starts from specific JWST clusters and estimates the black holes they could produce. The other simulates the process and connects it to a separate class of JWST sources. But both land on the same basic idea: in the crowded star-forming regions of the early universe, stellar collisions could make black holes far heavier than single stars normally leave behind.

Why It Matters

IMBHs, generally defined as black holes of roughly 100 to 100,000 solar masses, have long been the missing middle of the black hole population. Stellar-mass black holes and supermassive ones are well documented. The range in between has been much harder to fill. Universe Today points to two recent hints that it is populated: a roughly 8,200-solar-mass IMBH reported in the globular cluster Omega Centauri in a 2024 paper, and a 2020 gravitational-wave merger of 85 and 66 solar-mass black holes that left behind a black hole of about 142 solar masses.

The new work adds a formation pathway tied to objects that have actually been observed, not to hypothetical environments. If metal-poor clusters like those in the Cosmic Gems arc routinely produced seeds of a couple thousand solar masses within the first half-billion years, the "too big, too early" problem for supermassive black holes gets considerably easier. The authors describe such heavy seeds as highly beneficial for forming supermassive black holes at high redshift.

This does not show that heavy seeds did form in these clusters. The paper's estimates are analytic, and they depend on cluster properties inferred from lensed, heavily magnified images. The result shows that the clusters JWST is finding have masses, sizes and chemistry that make the process plausible. Confirming it will take observations that can detect the black holes themselves, or the signs of their growth, in clusters like these.