The James Webb Space Telescope may have identified the kind of stellar environment capable of producing the massive “seeds” that eventually grew into the universe’s earliest supermassive black holes. New analysis of two gravitationally lensed galaxies, the Cosmic Gems arc and Firefly Sparkle, points to dense, young star clusters as plausible factories for intermediate-mass black holes—and potentially the starting points for much larger monsters.
The JWST observations behind the finding reach back to an era when the universe was only a few hundred million years old. The Cosmic Gems arc was observed at roughly 460 million years after the Big Bang, while Firefly Sparkle appeared about 600 million years after it. Webb identified five young massive clusters in Cosmic Gems and 10 in Firefly Sparkle, giving astronomers unusually early examples of compact stellar systems.
The research, led by Viola Bocchi of Sapienza University of Rome and published in Astronomy & Astrophysics, estimates that these clusters could have produced intermediate-mass black holes through runaway stellar collisions. The models place the resulting objects between roughly 100 and 4,000 solar masses. That is substantially larger than the ordinary stellar-remnant black holes created by individual stars, although the lower end overlaps with possible remnants of the universe’s first Population III stars.
Cosmic Gems appears to offer the strongest conditions. Its clusters are exceptionally compact, with half-mass radii near one parsec, and could have generated black holes between approximately 1,600 and 2,700 solar masses. Firefly Sparkle’s clusters are more spread out and generally less massive, producing estimated black holes of about 180 to 810 solar masses. The difference matters because collision-driven growth depends heavily on how tightly stars are packed in a cluster’s core.
Metallicity is another key ingredient. These ancient clusters contain very little material forged by earlier generations of stars—less than about two percent of the Sun’s metallicity in the most favorable cases. That scarcity reduces mass lost through stellar winds, allowing the most massive stars to retain more material before colliding or collapsing. NASA describes this pathway as one route to “heavy seeds,” alongside the direct collapse of enormous stars or gas clouds; such seeds can begin with thousands to hundreds of thousands of solar masses and grow rapidly by consuming gas, dust, and stars.
The result does not mean Webb directly detected an intermediate-mass black hole inside either galaxy. Instead, it identifies a credible birthplace for one. The study uses analytic runaway-collision models, checked against direct cluster simulations, because the extreme densities make a full N-body treatment difficult. Unknowns such as internal rotation, cluster structure, the stellar mass distribution, and primordial mass segregation could all change the final masses—possibly making the black holes smaller, though a top-heavy population could have the opposite effect.
If the interpretation holds, these clusters could help explain why some black holes appear extraordinarily massive so early in cosmic history. They may also connect to Webb’s mysterious “little red dots,” whose spectra are often associated with rapidly growing black holes. The next step is to measure the clusters’ properties more precisely and search for signs of the black holes themselves, testing whether these compact, metal-poor stellar nurseries truly supplied the heavy seeds needed to build the first quasars and galactic-center supermassive black holes.
