Using computer simulations, astronomers have moved closer to solving the origin of globular clusters, some of the oldest stellar objects in the Universe. The key turned out to be the conditions under which the first such systems formed billions of years ago, particularly their rotation speed and gas content. Clusters that rotated slowly and were rich in gas most closely resemble those that surround the Milky Way today.
An Ancient Question
Among the oldest objects in the Universe are globular clusters. These dense concentrations of hundreds of thousands of stars have been orbiting galaxies such as the Milky Way for more than ten billion years.
Despite their great age and abundance, astronomers have still not been able to fully explain where they came from. For a long time, the dominant hypothesis was that such systems formed in the turbulent gas environment of the early Universe, but the specific conditions of their formation remained poorly understood.
Two Formation Scenarios
The researchers used the high-resolution MassiveBlackPS simulation to compare two types of young clusters in the early Universe. Some formed inside a rotating galactic disk, in a dense and turbulent region where most star formation takes place. Others appeared in quieter filamentary structures scattered through diffuse gas outside the galactic disk.
The authors figuratively called this second category cosmic wallflowers, by analogy with those who stay near the wall at a dance, apart from the general movement. The team was led by Flor van Donkelaar of the University of Cambridge, according to phys.org. A preprint describing the results was published on arXiv at the end of June, so the conclusions have not yet undergone formal peer review.
The Role of Rotation Speed
The key characteristic turned out to be the rate of rotation, which determines the later fate of newborn clusters. Disk systems are born in the already rapidly rotating environment of the parent disk and move just as quickly themselves.
Among the cosmic wallflowers, the range is much wider, from almost motionless systems to those rotating at high speed. The slowly rotating group has lower density, and its properties match those of modern globular clusters in our galaxy. The authors consider this group the most likely candidate for the ancestors of the Milky Way’s ancient systems.
Two Fates
The fast-rotating group of cosmic wallflowers dynamically resembles disk counterparts and is unlikely to survive long enough to become a globular cluster. Such dense systems are vulnerable to the tidal forces of the parent galaxy, which destroy them over billions of years. The authors do not rule out that these objects may have given rise to massive black holes through runaway stellar collisions in their cores.
One important pattern concerns the relationship between gas content and rotation rate. Gas-rich cosmic wallflowers turned out to be slow-rotating. If such systems retain their gas reserves and active star formation continues inside them, chemically diverse multi-population structures emerge, similar to those seen in modern globular clusters.
Limitations and Plans
The model reproduces clusters at the moment of birth and does not trace their later evolution, so the question of long-term survival remains unresolved for now. The team plans to analyze the gas dynamics around newborn systems in greater detail in order to determine what exactly sets their initial rotation pattern.
The study echoes recent discoveries made with the James Webb Space Telescope. At high redshifts, unexpectedly dense and massive stellar clusters have been detected, and the scenario of formation outside a galactic disk offers a natural explanation for them. The same mechanism may also explain where the seeds of supermassive black holes in young galaxies of the early Universe came from.
