Cosmic wallflowers: the circumgalactic origins of isolated ultra-compact star clusters at $z>7$

TL;DR

This work shows that dense, off-disc star clusters can form directly in circumgalactic filaments at $z>7$, providing an alternative to disc fragmentation for early cluster assembly. Using the MassiveBlackPS high-resolution zoom-in simulation, the authors identify 55 baryon-dominated, off-disc clusters formed via filament fragmentation, reaching extreme central densities comparable to JWST-detected compact clusters. Depending on metallicity and local baryon content, these clusters can seed intermediate-mass black holes through runaway collisions or survive as proto-globular clusters, potentially linking high-redshift clusters to present-day GCs and early SMBH seeds. The results align with JWST observations of compact clusters in lensing fields and predict CGM signatures for upcoming JWST and ALMA tests of off-disc cluster formation.

Abstract

The discovery of gravitationally lensed stellar clusters at high redshift with the James Webb Space Telescope (JWST) has revealed extremely compact, massive star-forming systems in galaxies at $z > 6$, providing a new window into early cluster formation. In this work, we investigate star cluster formation in the circumgalactic environments of gas-rich galaxies with stellar masses spanning between $\sim$$10^{8}$ - $10^{11}$ M$_{\odot}$ at $z > 7$, using the MassiveBlackPS cosmological hydrodynamical simulation with 2 pc resolution. We identify 55 baryon-dominated clusters forming outside galactic discs but within the virial radius of the primary halo. Star formation in these systems proceeds rapidly, reaching peak stellar surface densities above $10^{5}$ M$_{\odot}$ pc$^{-2}$, closely matching the compact clusters recently discovered by JWST in the lensed Cosmic Gems Arc at $z \approx 9.6$. Such extreme densities are a key pre-requisite to trigger runaway stellar collisions, indicating that a subset of our clusters would be a likely host of intermediate-mass black holes (IMBHs). We find that massive star clusters can form efficiently in the circumgalactic medium at early times through filament fragmentation, whereby high gas densities lead to rapid local collapse via a combination of thermal and gravitational instabilities. This formation pathway implies that some compact clusters formed in the quiet outskirts of forming galaxies rather than within their discs. Small variations in filament properties, including metallicity, density, and dark-matter content, influence the likelihood of a star cluster being able to form an IMBH seed. The formation of clusters in circumgalactic environments points to a potential evolutionary pathway connecting early off-disc clusters, present-day globular clusters, and the seeds of massive BHs.

Figures (11)

• Figure 1: Gas (left-hand panel) and stellar (right-hand panel) surface density maps of the simulated system at the final snapshot, $z \sim 7.6$. The maps show the full line-of-sight projection through the simulation volume. White crosses and numbered circles mark all identified clusters that satisfy the selection criteria described in the text, with the circles highlighting six randomly selected clusters (labeled 1--6) that are zoomed-in in Figure \ref{['fig:showclust2']}.
• Figure 2: Zoom-in views of the six randomly selected clusters highlighted in Figure \ref{['fig:showclust1']} with white, dashed, numbered circles. Each pair of panels shows the gas (left-hand side) and stellar (right-hand side) surface density maps centred on the cluster positions. In each panel, the left-hand side shows the gas surface density within a (1 kpc)$^2$ region, whereas the right-hand side presents a zoomed-in view of the stellar surface density within a (200 pc)$^2$ region.
• Figure 3: Top panel: gas fraction of the identified stellar clusters as a function of stellar mass. The dashed horizontal line marks a gas fraction of 10 per cent. Bottom panel: average SFR over the past 75 Myr for all identified stellar clusters versus stellar mass. The dashed, orange, horizontal lines indicate SFR levels of 1, 0.1, and 0.01 M$_{\sun}$ yr$^{-1}$, whereas the shaded region highlights clusters with SFRs below 0.01 M$_{\sun}$ yr$^{-1}$. The gray, dotted, vertical line denotes the mean stellar mass of the clusters in the Sunrise Arc at $z \sim 6$Vanzell:2023aa. In both panels, the colour bar represents the distance from the central halo.
• Figure 4: Correlation between stellar surface density, $\Sigma_{\star}$, and half-mass radius of the clusters (open purple circles). Gray dashed lines represent star clusters of equal mass, whereas the black contour lines shows the stellar density-radius relation for clusters in local galaxies from Brown:2021aa, using clusters from the LEGUS survey. The dark purple markers represent the stellar surface density and radius of clusters identified in the Cosmic Gems Arc at $z \sim 9.6$Adamo:2024aaMessa:2025aa, the Firefly Sparkle at $z \sim 8.3$Mowla:2024aa, and the Sunrise Arc at $z \sim 6.0$Vanzell:2023aa. The orange contour lines indicates the clusters found in the galactic discs in the work of Mayer:2025aa. Lastly, the vertical, black, dotted line corresponds to the softening length of the simulation.
• Figure 5: Stellar metallicity as a function of stellar surface density. Purple points mark the clusters identified in this work, while orange contour lines show the distribution of disc clusters from Mayer:2025aa.