Astronomers using computer simulations have investigated whether a class of star clusters nicknamed “cosmic wallflowers” could be the long-sought ancestors of the globular clusters we see orbiting galaxies today. Their paper, posted to the arXiv preprint server on June 25, suggests that where a cluster is born and how fast it spins may determine its fate.
Globular clusters are among the oldest objects in the universe. These dense, ancient balls of hundreds of thousands of stars have orbited galaxies like the Milky Way for more than 10 billion years. Yet despite their age and abundance, a basic question has never been fully answered: Where did they come from?
Astronomers have long suspected they formed in the violent, gas-rich environments of the early universe. But the exact conditions that allowed some clusters to survive billions of years of gravitational tug-of-war, while countless others were torn apart, have remained poorly understood.
This new study, led by Floor van Donkelaar of the University of Cambridge, alongside co-authors Lucio Mayer and Pedro R. Capelo, investigates the origin of globular clusters.
Born different
The researchers compared two types of star-forming clusters in the early universe using a high-resolution computer simulation called MassiveBlackPS. The first formed inside the rotating disk of a galaxy, the flat, turbulent region where most star formation happens. The second formed in quieter, more isolated filamentary structures in the diffuse gas surrounding galaxies, far from the disk itself. The researchers call these outsiders “cosmic wallflowers.”
They found that how fast a cluster spins plays a key role in determining whether it survives. Disk clusters form within an already rapidly spinning galactic environment and turn out to be fast-spinning. Cosmic wallflowers, by contrast, span a much wider range of dynamical states, from slowly rotating to rapidly spinning. Within this diversity, they found a worthy candidate.
The subset of cosmic wallflowers that rotates slowly is less dense and overlaps closely in its properties with present-day globular clusters. Researchers say these are the most plausible ancestors of the ancient clusters orbiting the Milky Way today.
The other subset spins faster and behaves more like disk clusters. It is unlikely to survive long enough to become a globular cluster. Clusters that rotate rapidly are more vulnerable to being shredded by their host galaxy’s tidal forces over time. These denser, more rapidly rotating systems are also natural candidates for runaway stellar collisions and possibly even the seeds of massive black holes.
The split
Further analysis also uncovered a link between gas content and rotation in cosmic wallflowers. Only the gas-rich wallflowers appeared to rotate slowly and match closely with the known globular clusters. Gas-poor ones rotate faster and are more likely to follow a different, shorter fate.
If these gas-rich, slowly rotating wallflowers retain their gas and continue forming stars, the researchers note, they would also naturally produce the kind of chemically complex, multigenerational stellar populations that are a hallmark of present-day globular clusters.
“The emergence of GC-like systems is therefore not a generic outcome of clustered star formation, but the result of specific conditions that are already established at early times,” the team writes in the paper.
The team cautions that the simulation shows clusters at birth rather than tracking their full evolution, so long-term survival remains to be confirmed. They plan to follow up with a closer look at the gas dynamics surrounding newly forming clusters to better understand what sets each cluster’s initial rotation pattern.
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Publication details
Floor van Donkelaar et al, Too shy to spin? Cosmic wallflowers as proto-globular clusters, arXiv (2026). DOI: 10.48550/arxiv.2606.27426
Journal information:
arXiv
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‘Cosmic wallflowers’ may hold the key to the origin of globular clusters (2026, July 7)
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