Tiny galaxies orbiting the Milky Way may hold clues to one of cosmology’s biggest mysteries.
Ultra-faint dwarf galaxies are among the smallest known galaxies orbiting the Milky Way. Astronomers have long viewed them as ancient remnants from the early cosmos. Now, researchers at the Oskar Klein Centre and the LYRA collaboration have used a powerful new set of simulations to show that these dim galaxies may reveal how conditions in the young Universe shaped which galaxies were able to grow and which never formed stars at all.
The study, published in Monthly Notices of the Royal Astronomical Society (MNRAS), was led by Azadeh Fattahi, Associate Professor at the Oskar Klein Centre (OKC), along with collaborators from Durham University and the University of Hawaii.
She explains the scale of the project: “In this work we presented a brand-new suite of cosmological simulations focused on the faintest galaxies in the Universe, with an unprecedented resolution. These are by far the largest sample of such galaxies ever simulated at these resolutions.”
Tiny Galaxies at the Edge of Understanding
Dwarf galaxies are much smaller than the Milky Way and form inside small dark matter halos predicted by standard cosmological models. The faintest examples are extremely fragile and sit near the edge of what scientists currently understand about galaxy formation and dark matter.
“The smallest galaxies are called ultra-faint dwarf galaxies, which are a million times less massive than the Milky Way or even smaller,” Fattahi says. “Due to their small size these galaxies have proven very difficult to model and simulate.”
The new simulations provide researchers with a much clearer and more systematic picture of how these galaxies formed over cosmic history.
“A useful analogy is to plants and crops and how the way they grow is sensitive to the weather conditions”, says Shaun Brown who led the study while working at OKC and Durham University. “In the same way that the yield of a crop in summer can indirectly tell you a lot about what the weather in spring must have been like, the properties of faint dwarf galaxies today can tell us a lot about the conditions, or weather, of the Universe at a much earlier time.”
A Window Into the Early Universe
The simulations are especially important because they do more than recreate faint dwarf galaxies. They also suggest that these nearby objects can reveal information about the Universe’s earliest “climate.” The team tested how different ideas about early radiation conditions affected whether small dark matter halos could form stars.
“In the paper we studied two different assumptions about the properties of the early Universe when it was less than 500 million years old, to understand the effect on the properties of these small galaxies today when the Universe is 13 billion years old,” Brown explains.
The results showed a strong effect in the smallest galaxies.
“We found that these small ultra-faint galaxies are very sensitive to these changes, while more massive galaxies, like our Milky Way, don’t really care,” he adds. “For the smallest galaxies, early conditions can decide whether they become visible galaxies, or remain starless dark matter halos.”
This sensitivity could help scientists test ideas about early-Universe physics with future observations.
“Excitingly, in the near future we will have data from the Vera C. Rubin Observatory which will be able to find many more of these ultra-faint dwarfs around the Milky Way,” says Fattahi.
Future Observations Could Test the Theory
Astronomers hope the Vera C. Rubin Observatory will uncover nearly all of the Milky Way’s satellite galaxies. According to the new study, those discoveries may also provide insights into conditions that existed shortly after the Big Bang.
“Our work suggests that these upcoming observations of the very local Universe will be able to constrain what the Universe at its infancy looked like, something we currently cannot directly access with other observations.”
The findings also connect to recent discoveries from the James Webb Space Telescope (JWST), which has detected unexpectedly massive and bright galaxies in the early Universe.
“The result is particularly relevant in light of recent discoveries of galaxies in the early Universe, by the James Webb Space Telescope (JWST), which is finding many surprises, in particular unexpectedly massive and bright galaxies in the early universe,” Fattahi notes.
If distant galaxies are challenging current theories about the early cosmos, nearby ultra-faint dwarf galaxies may offer another way to investigate what happened during that period.
Massive Simulations and Future Questions
Studying galaxies this faint required major computing power.
“Running these simulations is challenging, and extremely expensive in both time and computational resources,” Fattahi says.
Altogether, the simulations took more than six months to run. They also produced an enormous amount of data.
“The simulation also produces very large amounts of data (in total approximately 300 TB). This meant many of the old algorithms designed for smaller amounts of data needed updating and improving to effectively handle this new large amount of data.”
Most of the work was performed on COSMA 8, a supercomputer built for simulation-based research. COSMA 8 is hosted by Durham University’s Institute for Computational Cosmology on behalf of the UK’s DiRAC High Performance Computing Facility.
Fattahi’s team now plans to use the simulation suite to explore major unanswered questions in galaxy and structure formation. These include where the first generation of stars in the Universe might be found and what ultra-faint dwarf galaxies can reveal about the nature of dark matter.
Reference: “LYRA ultra-faints: the emergence of faint dwarf galaxies in the presence of an early Lyman–Werner background” by Shaun T Brown, Azadeh Fattahi, Thales A Gutcke, Sylvia Ploeckinger, Joaquin Sureda, Sownak Bose, Jessica E Doppel, Rüdiger Pakmor and Adrian Jenkins, 24 April 2026, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stag439
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