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

Using high-resolution cosmological simulations, this study reveals that massive, ultra-compact star clusters observed by JWST at $z>7$ can form efficiently in the circumgalactic medium through filament fragmentation, providing a potential evolutionary pathway for these systems to become present-day globular clusters and host intermediate-mass black holes.

Original authors: Floor van Donkelaar, Lucio Mayer, Pedro R. Capelo, Debora Sijacki, Angela Adamo

Published 2026-05-13

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CC BY 4.0

Original authors: Floor van Donkelaar, Lucio Mayer, Pedro R. Capelo, Debora Sijacki, Angela Adamo

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the early universe as a vast, chaotic construction site. Usually, we think of stars being born in the "cities" of the cosmos: the bright, swirling discs of young galaxies. But this paper suggests that some of the most massive and compact star clusters were actually born as "cosmic wallflowers"—lonely, isolated groups forming in the quiet, dark outskirts, far away from the main galactic party.

Here is a simple breakdown of what the researchers found, using everyday analogies:

1. The Setting: A Cosmic Construction Site

The researchers used a super-powerful computer simulation (like a virtual time machine) to look at the universe when it was very young (about 7 billion years before today). They were looking at massive galaxies that were still being built.

Usually, we expect stars to form in the "downtown" areas of these galaxies (the discs). However, the James Webb Space Telescope (JWST) recently spotted incredibly dense, compact star clusters in the middle of nowhere, far from the main galaxy. The scientists wanted to know: How did these lonely clusters get there?

2. The Discovery: "Cosmic Wallflowers"

The team found 55 of these isolated star clusters. They call them "cosmic wallflowers" because they formed outside the main galactic discs, sitting quietly in the gas streams (filaments) that feed galaxies.

The Analogy: Imagine a busy city (the galaxy) where most people live in apartments. But in this simulation, they found entire neighborhoods of people building houses in the empty fields and along the highways leading into the city, completely separate from the city center.
The Result: These clusters are incredibly dense. They pack so many stars into such a small space that they look like "stellar sardine cans." Their density matches exactly what JWST has recently seen in the distant universe.

3. How They Formed: The "River" Effect

Instead of forming from a spinning disc of gas (like a pizza dough being spun), these clusters formed because of filament fragmentation.

The Analogy: Think of a river flowing toward a lake. Sometimes, the water gets so thick and turbulent that chunks of the river break off and swirl into their own mini-vortices.
The Process: In the early universe, huge rivers of gas flowed toward galaxies. In certain spots, these gas rivers got so dense and cool that they collapsed under their own weight, instantly birthing these compact star clusters. It's like a river suddenly freezing into a solid block of ice in the middle of the flow.

4. The Two Paths: "The Monster" vs. "The Survivor"

The paper reveals that these clusters take one of two very different paths, depending on a few small details like how much "heavy metal" (chemical elements) is in the gas.

Path A: The Runaway Monster (IMBHs)
- The Scenario: In some clusters, the gas is rich enough in metals to cool down very fast. This makes the stars pack together so tightly that they start crashing into each other like cars in a pile-up.
- The Result: These collisions create a "runaway" effect, merging stars into one giant, super-massive star that eventually collapses into a Black Hole (specifically, an Intermediate-Mass Black Hole).
- The Analogy: It's like a mosh pit where everyone is so close that they merge into one giant person. The original "crowd" (the star cluster) disappears, leaving behind just the "giant" (the black hole).
Path B: The Survivor (Proto-Globular Clusters)
- The Scenario: In other clusters, the gas is a bit different (perhaps less metal-rich or influenced by dark matter). The stars form, but they don't crash into each other violently.
- The Result: These clusters survive as tight, bound groups of stars.
- The Analogy: This is like a dance troupe that forms a tight circle and stays together for billions of years. The paper suggests these might be the ancestors of the Globular Clusters we see in our own Milky Way today.

5. Why This Matters

This discovery changes how we think about the early universe.

Not Just City Dwellers: It proves that massive star clusters don't have to form inside the main galaxy. They can grow up in the "suburbs" (the cosmic filaments) and still become incredibly dense.
Black Hole Seeds: The "monster" clusters that turn into black holes could be the missing seeds that grew into the super-massive black holes we see in the centers of galaxies today.
Ancient Relics: The "survivor" clusters could be the ancient, fossilized star clusters we see in the sky right now.

Summary

The paper tells us that in the early universe, gas rivers flowing toward galaxies didn't just feed the main galaxy; they also broke off to form "cosmic wallflowers." Some of these wallflowers became so crowded they turned into black holes, while others survived as the tight, ancient star clusters we see today. It's a story of how the quiet outskirts of the universe played a crucial role in building the cosmic structures we see now.

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

Problem Statement
Recent observations by the James Webb Space Telescope (JWST) have revealed extremely compact, massive star-forming systems at redshifts $z \gtrsim 6–10$ , including those in the lensed Cosmic Gems Arc. While these systems share structural properties with present-day globular clusters (GCs), their formation mechanisms remain debated. Classical models often invoke formation within dark matter (DM) haloes, mergers, or thermal instabilities, but struggle to reconcile the lack of DM in modern GCs. Furthermore, while disc fragmentation is a known channel for cluster formation, growing evidence suggests that some compact stellar systems at high redshift are offset from galactic discs. The origin of these "off-disc" clusters and their potential role as progenitors of GCs or seeds for intermediate-mass black holes (IMBHs) requires investigation in high-resolution cosmological environments.

Methodology
The authors utilize the MassiveBlackPS cosmological hydrodynamical simulation, a high-resolution re-simulation of a sub-volume from the MassiveBlack simulation. The target region represents a massive, highly biased halo at $z \simeq 6–7$ , typical of hosts for bright quasars.

Resolution: The simulation employs smoothed-particle hydrodynamics (SPH) and $N$ -body methods (Gasoline2 code). Through gas-phase particle splitting, the final gas mass resolution reaches $2.4 \times 10^3 M_\odot$ , with a minimum gravitational softening of 2 pc.
Physics: Star formation (SF) and feedback follow Stinson et al. (2006) recipes with a Kroupa IMF. Cooling includes non-equilibrium H/He species and metal fine-structure cooling (linearly proportional to metallicity) under a redshift-dependent UV background.
Cluster Identification: The study focuses on the epoch $z=7.6$ $z = 7.6$ , approximately 6 Myr after a major ( $\sim 1:1.2$ $\sim 1 : 1.2$ ) merger. Using the AMIGA Halo Finder (AHF), the authors identify 55 stellar substructures satisfying strict criteria:
- Located $>2.5$ kpc from the central halo center (to exclude the dense central disc).
- Bound structures with effective radii $>4\times$ the softening length ( $>8$ pc).
- Stellar half-mass radius $<15$ pc and total stellar mass $<8 \times 10^7 M_\odot$ (excluding dwarf galaxies).
- Baryon-dominated (bound baryon fraction $>0.75$ ), ensuring negligible DM content.

Key Contributions and Results

Discovery of Circumgalactic Clusters ("Cosmic Wallflowers"):
The study identifies 55 baryon-dominated star clusters forming outside galactic discs but within the virial radius of the primary halo. These clusters form along accreting gas filaments rather than within the disc. They exhibit stellar masses between $10^{4.2}$ and $10^{7.4} M_\odot$ and are located at distances of 2.55 to 23.5 kpc from the halo center.
Structural Properties and JWST Comparison:
The identified clusters display extreme stellar surface densities, frequently exceeding $10^5 M_\odot \text{pc}^{-2}$ . These values closely match the compact clusters recently detected by JWST in the Cosmic Gems Arc ( $z \approx 9.6$ ) and other lensed systems. The clusters are gas-rich (gas fractions $>10\%$ for most), indicating they are in an early formation stage (proto-clusters) rather than evolved, gas-free systems.
Formation Mechanism: Filament Fragmentation:
The paper demonstrates that these clusters form via the gravitational fragmentation of gas filaments. Analysis of the gas mass ( $M_{\text{gas}}$ ) relative to the critical mass for collapse ( $M_{\text{crit}}$ ) shows that $M_{\text{gas}}/M_{\text{crit}} > 1$ for the majority of clusters, confirming gravitational instability. The process is driven by a combination of thermal and gravitational instabilities as gas cools and condenses within the filaments, leading to rapid local collapse.
IMBH Progenitors and Evolutionary Pathways:
- Runaway Collisions: A subset of the densest clusters (surface densities $>10^4 M_\odot \text{pc}^{-2}$ and metallicities $Z_* < 0.5 Z_\odot$ ) possess conditions favorable for runaway stellar collisions, potentially forming IMBHs with masses up to $\sim 10^4 M_\odot$ .
- Dynamical Friction: The authors estimate that while massive clusters may migrate toward the galactic center via dynamical friction within a Hubble time, many will not reach the center before being disrupted or losing mass.
- Divergent Fates: The study proposes two evolutionary pathways based on local environmental conditions:
  - IMBH Seeds: Clusters forming in regions with slightly higher baryon fractions and higher metallicities (enhancing cooling) undergo rapid collapse, leading to runaway collisions and IMBH formation. These systems likely do not survive as long-lived clusters.
  - Proto-GCs: Clusters forming in slightly more DM-influenced regions with lower metallicities avoid the runaway regime, allowing them to survive as bound (proto-)globular clusters.
Role of Metallicity:
Stellar metallicity varies widely ( $0.00035 \lesssim Z_*/Z_\odot \lesssim 1.49$ ), with the densest clusters forming in regions with metallicities higher than the global mean. Higher metallicity promotes efficient cooling, allowing gas to reach higher densities before feedback disperses it, thereby favoring the formation of compact, bound systems and increasing the likelihood of IMBH formation.

Significance and Claims
The paper claims that filament fragmentation in the circumgalactic medium (CGM) is a viable and efficient mechanism for producing dense, ultra-compact star clusters at $z > 7$ . This pathway offers an alternative to in-disc fragmentation for explaining the "offset" clusters observed by JWST.

The authors posit that these "cosmic wallflowers" represent a fundamental mode of early stellar assembly. Crucially, the study suggests a potential evolutionary link connecting:

Early off-disc clusters formed in filamentary environments.
Present-day globular clusters (those that avoid runaway collapse).
The seeds of massive black holes (those that undergo runaway collapse).

The work emphasizes that small variations in filament properties—specifically metallicity, density, and local dark matter content—determine whether a cluster evolves into a long-lived GC or a progenitor of an IMBH. The authors conclude that a significant fraction of today's GCs and early SMBH seeds may have originated in the "quiet outskirts" of forming galaxies rather than within their bright discs. They note that while their conclusions are robust regarding the cooling and condensation of filaments, the specific outcomes are sensitive to the treatment of small-scale star formation, feedback, and radiative transfer, which warrant further investigation.