For generations, astronomers have pondered the origins of globular clusters, which are among the universe’s oldest and densest star systems. Research led by the University of Surrey and published in Nature has finally answered the puzzle with the help of thorough simulations, while also revealing a new kind of object that might already exist in the Milky Way.
A globular cluster (white concentration of stars) naturally emerges in the high-resolution EDGE simulations. These simulations also predict the existence of a new class of object: globular cluster-like dwarfs. These new objects form similarly to globular clusters, but in their own dark matter halo. The nearby Reticulum II dwarf galaxy may be such an object that has been hiding in plain sight in our cosmic backyard. If so, it promises unprecedented constraints on the nature of dark matter and a new place to hunt for the first metal-free stars. Image Credit: University of Surrey, Matt Orkney, Andrew Pontzen & Ethan Taylor
Globular clusters are dense groups of hundreds of thousands to millions of stars that orbit galaxies, including the Milky Way. Unlike galaxies, they contain no dark matter, and their stars are extraordinarily consistent in age and chemical composition - characteristics that have sparked debate among scientists since their discovery in the seventeenth century.
Surrey researchers used ultra-high-resolution simulations to trace the Universe’s 13.8-billion-year history in unprecedented detail, allowing them to observe globular clusters grow in real time within their virtual cosmos, known as EDGE. The simulations reveal multiple pathways for their formation, as well as the unexpected appearance of a new kind of star system, “globular cluster-like dwarfs”, with features intermediate between globular clusters and dwarf galaxies.
The formation of globular clusters has been a mystery for hundreds of years, so being able to add additional context surrounding how they form is amazing. We were able to do this in our EDGE simulations without having to add anything special to make them appear, and it just brings the simulations that extra level of realism. Additionally, being able to find a new class of object in the simulations is very exciting, especially since we have already identified a handful of candidates which exist in our very own Milky Way.
Ethan Taylor, Postgraduate Research Student, University of Surrey
Researchers collaborated with Durham University, the University of Bath, the University of Hertfordshire, Carnegie Observatories, and the American Museum of Natural History in the United States, Lund University in Sweden, and the University of Barcelona in Spain to run the EDGE simulations over several years on the UK’s DiRAC National Supercomputer facility.
To put the size in perspective, the biggest simulations would take decades to complete on a typical or high-end laptop. These models not only replicated genuine globular clusters and dwarf galaxies, but they also projected a previously unknown class of object.
Dark matter makes up the vast majority of mass in typical dwarf galaxies, often outweighing their stars and gas by a factor of nearly a thousand. However, the newly identified "globular cluster-like dwarfs" appear similar to regular star clusters at first glance. Despite their appearance, they contain a significant amount of dark matter, suggesting that telescopes may have already observed these objects in the real universe but mistakenly classified them as ordinary globular clusters. This minor difference would put them in a unique position to investigate both dark matter and cluster formation.
Several known Milky Way satellites, including the “ultra-faint” dwarf galaxy Reticulum II, are potential candidates. If verified, they might serve as prime sites for the hunt for pure, metal-free stars formed in the early Universe, as well as fresh locations to test theories for the elusive “dark matter.”
The EDGE project set out to build the most realistic simulation of the very smallest galaxies in the Universe – one that could follow all 13.8 billion years of its history while still zooming in on the tiny details, like the blast from a single exploding star. It took years to run on the UK’s DiRAC National Supercomputer, but the payoff has been extraordinary. At a resolution of just 10 light years, fine enough to capture the effects of individual supernovae, we’ve been able to show that globular clusters can form in at least two different ways, both without dark matter.
Justin Read, Professor and Early Career Researcher Lead, University of Surrey
The next step is to prove the presence of these globular cluster-like dwarfs by targeted observations with telescopes like the James Webb Space Telescope and future deep spectroscopic surveys. If they do, it might provide astronomers with new tools to test dark matter ideas, as well as some of the best possibilities of discovering the Universe's first generation of “metal-free” stars.
Journal Reference:
Taylor, E, D., et al. (2025) The emergence of globular clusters and globular-cluster-like dwarfs. Nature. doi.org/10.1038/s41586-025-09494-x