Massive star clusters detected by JWST as natural birth places to form intermediate-mass black holes

Abstract

The James Webb Space Telescope (JWST) has detected, through gravitational lensing, several young massive star clusters (YMCs), which are considered as relevant building blocks of high redshift galaxies. In this work, we show how a significant fraction of these YMCs could act as relevant birth places for intermediate-mass black holes. We first consider the formation of massive clusters and show that the population of YMCs is consistent with a steep mass-radius relation, which includes a relevant spread of roughly an order of magnitude. We pursue a comparison of this population with young star clusters in the local Universe and Milky Way globular clusters, including an analysis of the characteristic timescales. The YMCs show a wide spread over these properties, but include systems with both short relaxation times as well as relatively short collision timescales, implying they could go through efficient core collapse, which would lead to runaway collisions. We provide quantitative estimates of the sizes of the clusters that could efficiently form intermediate-mass black holes through a runaway collision-based channel, suggesting that these roughly correspond to the systems beyond the $1σ$ scatter in the mass-radius relation. This implies a fraction of ~16% of YMCs as candidates to form intermediate-mass black holes. We show that above a mass limit of ~6x10^6 M_sun, compact star clusters are likely to retain gas even in the presence of strong supernova feedback, altering the dynamics in the central core and providing the possibility to rapidly grow the central object both via gas dynamical friction and Bondi accretion. Finally, we consider the possibility of a gas-dominated regime, in which strong gravitational torques may inhibit star cluster formation and instead directly form a high-mass black holes, as suggested to have occurred in the infinity galaxy.

Figures (5)

• Figure 1: YSCs in the local Universe Brown2021 and the YMCs detected by JWST in the mass-radius diagram. The Marks2012 relation (dashed line) provides a good description for the more compact clusters within the sample, while the Grudic2023 relation (solid) line provides a better description for the typical clusters. The $1\sigma$ scatter reported by Grudic2023 corresponds to an order-of-magnitude variation of the radii, implying that the observed systems are consistent with the proposed relation.
• Figure 2: Populations of YSCs Brown2021, GCs Baumgardt2018Vergara2024, YMCs Vanzella2022Vanzella2023Adamo2024Mowla2024 and star-forminig clumps in high-redshift galaxies Fujimoto2025Nakane2025 in the mass vs half-mass-radius diagram. We show them together with several characteristic timescales which are evaluated for a typical value of $1.0$ Gyr (dashed lines) and minimum and maximum values of $0.1$ Gyr and $14$ Gyr, respectively. This includes the evaporation timescale (shaded purple), the dynamical friction inspiral timescale (shaded orange) and the collision timescale in the inner core (shaded green). We further provide the relaxation time at $14$ Gyr (blue dashed line) and the global collision time at $1.0$ Gyr (black dashed line). The minimum cluster mass to survive tidal disruption for $14$ Gyr is indicated via the red shaded area for a galactic radius of $10$ kpc, and for a galactic radius of $1$ kpc via the pink shaded area, assuming a circular velocity of $V_G=200$ km/s and $\Psi$ values between $1$ and $5$. Star cluster simulations are indicated as crosses, with the size of the cross indicating the mass of the massive black hole formed in the simulations of Vergara2025aVergara2025bMapelli2016Rizzuto2021Arca2023Wang2015Wang2021Kamlah2022Wang2024.
• Figure 3: Assuming collision-based black hole formation, we provide the required evolutionary timescale of stellar clusters as a function of their radius to form a seed black hole with a mass as indicated by the different lines, considering seed masses from $10^3$ M$_\odot$ up to $10^6$ M$_\odot$. The seed mass is translated into a cluster mass via Eq. \ref{['seed']}, and Eq. \ref{['crit']} is then employed to derive the relation between cluster radius and evolutionary time, assuming that the average mass per star corresponds to a solar mass and the average radius per star to a solar radius.
• Figure 4: Top panel: Efficiency to form a central massive object (ratio of mass in that object over total mass) as a function of $M/(RT)$ to approximate the gravitational focusing parameter $\Phi$ as introduced in Eq. \ref{['phi']}. Bottom panel: Here we show the efficiency to form a central massive object as a function of gas mass divided by thermal Jeans mass. In these plots we consider data points from simulations providing detailed models including collisions and accretion in massive gas-dominated clusters at sub-solar metallicities Chon2020Reinoso2023Solar2025.
• Figure 5: Summary of the black hole formation pathways discussed in this paper, describing the purely collisional regime (left), the case of gas retention in a massive compact cluster (mid panel) and the gas-dominated regime (right panel).