🔭 astrophysics

Too shy to spin? Cosmic wallflowers as proto-globular clusters

This study of high-redshift simulations reveals that isolated "cosmic wallflower" clusters forming in filaments are weakly rotating and gas-rich, making them natural proto-globular cluster candidates, whereas disc-formed clusters are strongly rotation-dominated and distinct from the observed globular cluster population.

Original authors: Floor van Donkelaar, Lucio Mayer, Pedro R. Capelo

Published 2026-06-29

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CC BY 4.0

Original authors: Floor van Donkelaar, Lucio Mayer, Pedro R. Capelo

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 massive, chaotic dance party. Stars are the dancers, and they don't just form randomly; they form in groups called clusters. For a long time, astronomers wondered: "Where do the most famous, long-lived dancers (called Globular Clusters) come from? Do they start out as the life of the party, or do they start as the shy wallflowers?"

This paper investigates that exact question by looking at a high-resolution computer simulation of the universe when it was very young (about 7.6 billion years ago).

Here is the breakdown of their findings using simple analogies:

1. The Two Types of Dance Floors

The researchers looked at two different places where star clusters were forming:

The "Disc" Dancers: These clusters formed inside the swirling, flat discs of young galaxies. Think of this like a crowded, spinning dance floor where everyone is already moving in a circle. Because the whole floor is spinning, the dancers (stars) here are fast and energetic. They have high "rotational velocity" (they spin fast) and are strongly supported by that spin.
The "Cosmic Wallflowers": These clusters formed in the isolated, quiet spaces between galaxies, along cosmic filaments (like invisible bridges of gas). The paper calls them "wallflowers" because, like a shy person at a party who isn't dancing, these clusters are isolated and spinning very slowly.

2. The Big Discovery: A Clear Split

When the scientists measured how fast these clusters were spinning versus how much they were jiggling randomly (a concept called "rotational support"), they found a clear separation:

Disc Clusters: They are all over the place in terms of speed, but they are generally fast spinners. They are like a group of dancers who learned to waltz together.
Wallflowers: They are mostly slow spinners. However, they are a mixed bag. Some are very slow and jiggly (pressure-supported), while a smaller group is actually spinning a bit faster.

3. The "Wallflowers" are the Ancestors of Globular Clusters

The most exciting part of the paper is what happens when they compare these ancient clusters to the Globular Clusters we see in our own Milky Way today.

The Match: A specific group of the "Cosmic Wallflowers" (the slow-spinning, gas-rich ones) looks exactly like the ancient Globular Clusters we see today. They have the same slow spin and the same density.
The Mismatch: The clusters that formed inside the galaxy discs are too fast and too dense. They don't look like the Globular Clusters we know.

The Analogy: Imagine you are trying to find the ancestor of a specific type of old, slow-moving turtle. You find two groups of baby turtles. One group is sprinting on a treadmill (the Disc clusters). The other group is slowly crawling in the mud (the Wallflowers). The paper concludes that the slow crawlers are the ones that will grow up to be the famous, slow-moving turtles we see today. The sprinters will likely evolve into something else entirely.

4. Why Are the Wallflowers Special?

The paper suggests a few reasons why these "wallflowers" are the perfect candidates for becoming Globular Clusters:

They are shy (Slow): Because they formed in quiet, isolated areas, they didn't inherit a lot of spin. This low spin is a key trait of the Globular Clusters we see today.
They are full of gas: The slow-spinning wallflowers are still holding onto a lot of gas (the fuel for making stars). This gas helps keep them together and might explain why they have multiple generations of stars (a common trait in Globular Clusters).
They are safe: Because they are far away from the center of the galaxy and not spinning wildly, they are less likely to be torn apart or dragged into the center of the galaxy by gravity. They are "safe" enough to survive for billions of years.

5. The "Double Life" of Wallflowers

The paper also notes that not all wallflowers are the same.

Group A (The Proto-Globulars): The slow, gas-rich, low-density ones. These are the ones likely to become the Globular Clusters we see today.
Group B (The Black Hole Seeds): The denser, faster-spinning wallflowers. These are likely to collapse in on themselves and become the seeds for massive black holes, rather than becoming star clusters.

The Bottom Line

The universe is a big party. Some stars form on the main dance floor (galaxy discs), spinning fast and destined for a different fate. Others form in the quiet corners (cosmic filaments), spinning slowly and holding onto their gas.

This paper argues that the slow, shy "wallflowers" in the corners are the natural ancestors of the Globular Clusters we see today. Their environment and their lack of spin set them on a path to survive for billions of years, eventually becoming the ancient, stable clusters we observe now. The fast-spinning disc clusters, meanwhile, are likely to evolve into something completely different.

Problem Statement
The formation and evolutionary pathways of globular clusters (GCs) remain a central question in galaxy assembly. While recent observations by the James Webb Space Telescope (JWST) have identified compact, dense star clusters at high redshifts ( $z \gtrsim 6$ ) resembling present-day GCs, the specific environmental conditions that allow these systems to survive and evolve into long-lived GCs are not fully understood. A key uncertainty lies in whether the dynamical state of a cluster at formation—specifically its rotational properties and internal kinematics—is imprinted by its birth environment (e.g., galactic discs versus isolated circumgalactic media) and whether this initial state dictates its long-term survival. Furthermore, the role of the baryonic gas component in setting these initial dynamical conditions requires clarification.

Methodology
The authors utilize the high-resolution isolated hydrodynamical simulation MassiveBlackPS, run with the Gasoline2 smoothed-particle hydrodynamics and $N$ -body code. The simulation focuses on a massive halo at $z \approx 7.6$ , following a major merger over $\sim 60$ Myr, with a specific analysis of the final $\sim 6$ Myr. The study compares two distinct populations of star-forming clusters identified within the simulation:

Disc Clusters: Systems forming via gravitational instability and fragmentation within the central galactic disc (Halo 0).
Cosmic Wallflowers (CWs): Isolated circumgalactic systems forming along filaments within the virial radius but outside the main galactic disc.

For each cluster, the authors measure internal kinematics within a spherical aperture of $2r_{half}$ (where $r_{half}$ is the stellar half-mass radius). Key metrics include the stellar rotational velocity ( $v_{rot}$ ), stellar velocity dispersion ( $\sigma$ ), and the rotational support ratio ( $v/\sigma$ ). They also analyze gas kinematics, specifically the co-rotating mass fraction ( $f_{corot}$ ) and the alignment between gas and stellar angular momentum vectors. Finally, the authors estimate the dynamical friction (DF) time-scale ( $\tau_{DF}$ ) to assess the likelihood of these clusters migrating into the central galaxy.

Key Contributions and Results
The study reveals a clear dynamical separation between the two formation channels, suggesting that the formation environment leaves a distinct imprint on cluster kinematics:

Kinematic Dichotomy: Disc clusters are strongly rotation-dominated, exhibiting high $v_{rot}$ (extending to $>100$ km s $^{-1}$ ) and low $v/\sigma$ . In contrast, the CW population spans a wide range of dynamical states, from dispersion-supported to rotation-dominated, but systematically possesses lower rotational velocities.
GC-like Candidates: A specific subset of low- $v_{rot}$ CWs ( $v_{rot} < 10$ km s $^{-1}$ ) overlaps significantly with the observed kinematic and structural properties (stellar surface density and $v/\sigma$ ) of present-day Milky Way GCs. Disc clusters remain offset from this region, suggesting they follow a different evolutionary trajectory.
Rotational Coherence: The CW population exhibits tight coupling between gas and stellar rotation, with high co-rotating mass fractions ( $f_{corot} \approx 0.81$ for stars, $0.98$ for gas) and small misalignment angles ( $\sim 3^\circ$ ). This indicates that the measured rotation in CWs traces coherent internal motion rather than numerical noise. Conversely, disc clusters show weak coupling between gas and stellar angular momentum, likely due to older stellar populations and complex disc interactions.
Internal Substructure of CWs: The CW population is not homogeneous. It separates into two regimes:
1. Low-density, weakly rotating systems: These overlap with GC properties, possess longer dynamical friction time-scales, and are less likely to rapidly sink into the central galaxy.
2. High-density, more rotationally supported systems: These likely follow a pathway toward runaway stellar collisions and the formation of intermediate-mass black hole (IMBH) seeds, similar to the densest CWs identified in previous work.
Gas Fraction Correlation: Gas content correlates with dynamical state in CWs but not in disc clusters. Gas-rich CWs preferentially occupy the low-rotation, GC-like regime, while gas-poor systems extend to higher $v_{rot}$ . This suggests that gas retention is linked to the initial dynamical state and potential evolutionary path.

Significance and Claims
The paper argues that the environment and baryonic content play a decisive role in setting the initial dynamical state of star-forming clusters. The authors propose that weakly rotating, gas-rich cosmic wallflowers are natural candidates for proto-globular clusters. These systems emerge with dynamical properties (low $v_{rot}$ , coherent gas structure, and longer survival time-scales against orbital decay) that align with the early stages of GC formation.

The study concludes that the emergence of GC-like systems is not a generic outcome of all clustered star formation. Instead, it is the result of specific conditions found in isolated circumgalactic environments, where clusters form with low angular momentum and retain gas, potentially evolving into present-day GCs through subsequent angular momentum loss and dynamical heating. In contrast, clusters forming in galactic discs are rotation-dominated and likely follow alternative evolutionary pathways, potentially failing to survive as long-lived GC analogues. The authors note that while these results provide a coherent framework, the systems analyzed are at the formation stage, and their long-term collisional evolution to $z=0$ is not directly simulated.