The James Webb Space Telescope (JWST) has found many young massive star clusters (YMCs) at early times in the Universe. In this paper the authors argue that a noticeable fraction of those clusters could form intermediate‑mass black holes (IMBHs). Two main routes are considered: runaway collisions of stars in a very dense cluster core, and rapid growth by accreting gas that the cluster keeps despite supernova explosions.

The team first examined how massive clusters form and how their sizes scale with mass. They find the YMC population is consistent with a steep mass–radius relation (for example one fit from simulations gives Rcluster ≈ 1.4 pc × (Mcluster/10^4 M⊙)^0.25) but with a large scatter of about an order of magnitude. They compared these distant clusters with young clusters in the nearby Universe and with Milky Way globular clusters. Some YMCs show very short relaxation times (the time for stars to redistribute their energies) and short collision times, conditions that let the core collapse and trigger runaway stellar collisions.

Using these timescales and the observed spread in sizes, the authors estimate which clusters would be compact enough for the collision route to work. They find the compact systems that lie beyond the 1σ scatter of the mass–radius relation make up about 16% of the YMCs and are candidates to form IMBHs through collisions. They also identify a mass threshold near 6 × 10^6 solar masses. Above that mass, compact clusters are likely to retain gas even with strong supernova feedback. That retained gas can change the dynamics in the center and let any central object grow quickly by gas dynamical friction (gas pulling objects inward) and Bondi accretion (steady gas inflow onto a compact object).

The authors further consider an extreme, gas‑dominated regime. In that case strong gravitational torques in a dense gas flow might prevent a normal star cluster from forming at all and instead build a high‑mass black hole directly. They mention this as a possible explanation for claims about the so‑called “infinity” galaxy. The paper also notes that numerical simulations and recent work on mass loss during collisions help make the collision channel more robust, but the exact outcomes depend on many details.