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Resolved internal dynamics of a proto–globular cluster at cosmic dawn

Using high-resolution cosmological simulations, researchers demonstrate that a proto-globular cluster with ordered internal rotation and properties matching recent JWST observations can form rapidly at cosmic dawn (z = 10.5), revealing that these ancient systems emerge as dynamically structured entities from the very beginning of their assembly.

Original authors: Raffaele Pascale, Francesco Calura, Enrico Vesperini, Alessandro Della Croce, Joki Rosdahl, Alessandro Lupi

Published 2026-06-25

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CC BY 4.0

Original authors: Raffaele Pascale, Francesco Calura, Enrico Vesperini, Alessandro Della Croce, Joki Rosdahl, Alessandro Lupi

Original paper licensed under CC BY 4.0 (https://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 trying to understand how a massive, ancient city was built, but all you have are blurry, low-resolution satellite photos. You can see the general area where the city exists, but you can't see the individual buildings, the streets, or how the people moved around when the city was first being constructed. This has been the problem for astronomers studying Globular Clusters—these are incredibly dense, ancient "cities" of stars that orbit our galaxy. We know they are old and heavy, but we've never been able to watch them form in a computer simulation with enough detail to see their internal structure.

This paper is like upgrading from a blurry satellite photo to a high-definition, 3D movie that lets us watch the construction of one of these stellar cities in real-time.

The Super-Resolution Camera

The researchers used a special set of computer simulations called SIEGE (Simulating the Environment where Globular Clusters Emerged). Think of most previous simulations as trying to build a model of a city using only a few large blocks of clay. You can see the shape, but you can't see the windows or the doors.

This new simulation is different. It uses "sub-parsec" resolution. To put that in perspective, a "parsec" is a huge distance in space. Breaking it down to a "sub-parsec" level is like zooming in so close that every single "brick" in the model represents one individual star, rather than a whole neighborhood of them. This allowed the team to see the "internal dynamics"—how the stars move, spin, and interact with each other—right from the moment the cluster was born.

The Rapid Construction Site

The simulation focused on a time in the early universe, about 13 billion years ago (when the universe was only a few hundred million years old). Here is what they saw happen:

The Collapse: Imagine a massive cloud of gas, dense enough to be a cosmic pressure cooker. Suddenly, it collapses. It's not a slow, gentle process; it's a violent, rapid crash.
The Merge: Two dense clumps of stars, which had formed nearby, crashed into each other and merged. It was like two busy construction crews suddenly joining forces to build one massive skyscraper.
The "Starburst": In a very short time—just about 3 million years (which is a blink of an eye in cosmic time)—this system assembled almost all of its stars. It reached a mass of about 200,000 suns.
The Cleanup: As the stars formed, they blew away the remaining gas (the "construction materials") with powerful winds. The construction site was cleared out quickly, leaving behind a tight, compact ball of stars with almost no gas left.

The Result: A Spinning, Dense City

What makes this discovery special is what they saw inside this newly formed cluster:

It was already organized: Unlike a pile of random rocks, this cluster had ordered rotation. It was spinning like a top. The simulation showed that the stars weren't just moving randomly; they were following a coherent pattern, preserving the "memory" of how the gas was spinning before it collapsed.
It matched the "JWST" sightings: Recently, the James Webb Space Telescope (JWST) has spotted mysterious, tiny, super-dense star clusters in the early universe. Scientists weren't sure if these were the "babies" of the globular clusters we see today. This simulation shows that the object they created looks exactly like those JWST sightings. It has the same size, density, and mass.
No Dark Matter "Skeleton": Usually, galaxies are held together by a huge, invisible skeleton of "dark matter." However, in this specific cluster, the stars became so dense and dominant that they took over. Inside the cluster, the stars are the boss, not the dark matter. The dark matter is still there on the outside, but the inner city is made entirely of stars.

The Big Picture

The paper claims that we finally have a "fossil record" of how these clusters were born. Because the simulation was so detailed, they could see that these clusters didn't need millions of years to form. They could pop into existence as fully formed, spinning, dense systems in just a few million years.

In short, the authors have built the first high-definition "birth certificate" for a globular cluster. They showed that these ancient stellar cities can form rapidly in the early universe, spin up with a specific rhythm, and look exactly like the mysterious objects we are currently spotting with our most powerful telescopes. It proves that the complex structure of these clusters was imprinted on them the very moment they were born.

Technical Summary: Resolved Internal Dynamics of a Proto–Globular Cluster at Cosmic Dawn

Problem Statement
The formation mechanism of globular clusters (GCs) remains a major unresolved problem in astrophysics. While GCs are known to be among the oldest, densest, and most metal-poor stellar systems in the Universe, current cosmological simulations have failed to resolve their internal dynamical structure at birth. Existing models typically treat stars as unresolved populations or sample compact systems with a limited number of particles. This limitation prevents a reliable characterization of internal structure, kinematics, and the specific physical processes driving their assembly. Consequently, no single formation scenario has yet provided a comprehensive explanation for the structural, chemical, and dynamical characteristics of GCs.

Methodology
To overcome spatial resolution and stellar sampling limitations, the authors utilized the SIEGE (Simulating the Environment where Globular Clusters Emerged) project. This suite employs cosmological zoom-in hydrodynamical simulations using the adaptive mesh refinement code RAMSES.

Key methodological features include:

Resolution: The simulations achieve a sub-parsec spatial resolution of approximately 0.4 pc (specifically $\simeq 0.3 h^{-1}$ pc at $z=6.14$ ).
Star-by-Star Sampling: Unlike standard simulations where stellar particles represent stellar populations, this model generates stars individually. Each stellar particle represents a single star drawn from a Kroupa initial mass function (0.1–100 $M_\odot$ ). This approach captures the stochastic nature of stellar feedback, particularly in low-mass systems.
Physics: The simulation includes gas cooling (hydrogen, helium, metal lines), a redshift-dependent UV background, and a high-efficiency star formation model ( $\epsilon_{SFE} = 1$ ) occurring in dense, cold gas ( $T < 2 \times 10^4$ K, densities up to $\simeq 10^5$ cm $^{-3}$ ).
Feedback: Feedback is implemented at the individual star level. Massive stars (8–40 $M_\odot$ ) inject mass and energy via stellar winds during their pre-supernova phase and via Type II supernovae (10 $^{51}$ erg) at the end of their lives.
Target: The study focuses on a specific massive proto-GC within a dwarf galaxy halo (virial mass $\sim 4 \times 10^9 M_\odot$ ) evolving from redshift $z \simeq 10.5$ down to $z = 6.14$ .

Key Results
The simulation successfully resolves the formation and internal dynamics of a proto-GC with a final mass of $\simeq 2 \times 10^5 M_\odot$ :

Rapid Assembly: The system forms through a single, short-lived event starting at $z \simeq 10.5$ . It assembles the majority of its stellar mass within approximately 3 Myr via the collapse of dense gas and the merging of two nearby dense stellar clumps.
Gas Depletion and Quenching: The system rapidly becomes gas-poor and star-dominated. Star formation is quenched not by supernovae (which occur later), but by the rapid consumption of the dense gas reservoir and heating by stellar winds. This leads to a sharp decline in the star formation rate after the initial burst.
Structural Properties: The resulting system is compact, confined within $\simeq 6$ pc, with central projected stellar surface densities of $\sim 10^4 M_\odot$ pc $^{-2}$ . These properties (density, size, mass) overlap significantly with compact stellar systems recently observed by JWST at high redshift.
Internal Kinematics: For the first time in a fully cosmological simulation, the internal dynamical structure of a forming proto-GC is resolved. The system develops ordered internal rotation shortly after assembly.
- The total angular momentum increases rapidly during assembly, peaks shortly after formation, and then gradually declines due to redistribution.
- Radial profiles at $\simeq 22$ Myr show differential rotation: inner regions are dominated by velocity dispersion ( $\sigma_\phi > v_\phi$ ), while outer regions exhibit coherent large-scale rotational patterns.
Dark Matter Environment: Although the proto-GC forms within a dark-matter-rich environment, its inner regions (out to $\sim 1.5 R_e \approx 25$ pc) are dominated by stars, with a stellar-to-dark-matter mass ratio reaching $\sim 5$ . The dark matter component remains diffuse on larger scales.

Significance and Claims
The paper claims that these results demonstrate that globular clusters can emerge as dynamically structured stellar systems already at birth. Key implications include:

Fossil Record: The internal kinematics of proto-GCs, specifically the presence of ordered rotation, may preserve a fossil record of the physical conditions (angular momentum of gas and stellar structures) under which they formed.
Formation Mechanism: GCs can form rapidly in extreme high-redshift environments without requiring a dominant dark-matter component on cluster scales, though the surrounding dark matter sets the large-scale conditions for collapse.
Simulation Capability: The study proves that cosmological simulations can directly capture GC formation when both parsec-scale gas collapse and individual stellar populations are resolved.

The authors conclude that the emergence of a compact, rotating, and dynamically structured proto-GC at cosmic dawn supports a formation picture where the main structural and dynamical properties of GCs are imprinted during the earliest stages of their evolution.

The Super-Resolution Camera

The Rapid Construction Site

The Result: A Spinning, Dense City

The Big Picture

Technical Summary: Resolved Internal Dynamics of a Proto–Globular Cluster at Cosmic Dawn

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