Azadeh Fattahi, Associate Professor at the Oskar Klein Centre (OKC) and leader of the research group, headed the work in collaboration with teams from Durham University and the University of Hawaii. The results are published in Monthly Notices of the Royal Astronomical Society.
"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," Fattahi said.
Dwarf galaxies are often described as small cousins of the Milky Way. They form in small dark matter halos predicted by the standard model of cosmology. The faintest examples are extreme in both size and fragility, lying at the boundary of knowledge 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. Due to their small size these galaxies have proven very difficult to model and simulate," Fattahi added.
Shaun Brown, who led the study while working at OKC and Durham University, offered a down-to-earth analogy for the research. "A useful analogy is to plants and crops and how the way they grow is sensitive to the weather conditions. 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."
The simulations go beyond simply reproducing faint dwarf galaxies - they suggest these local objects can serve as a probe of the Universe's earliest climate. The team examined how different assumptions about the early radiation environment influenced which small dark matter halos managed to form stars at all.
"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 explained.
The results were striking. "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. For the smallest galaxies, early conditions can decide whether they become visible galaxies - or remain starless dark matter halos," Brown said.
That sensitivity opens a direct path to testing early-Universe physics with upcoming observations. The Vera C. Rubin Observatory is expected to find many more ultra-faint dwarfs around the Milky Way in the near future, providing a near-complete census of Milky Way satellite galaxies.
"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," Fattahi said.
The result is also relevant in light of recent James Webb Space Telescope discoveries of unexpectedly massive and bright galaxies in the early Universe. Local ultra-faint dwarfs - relics from the same epoch - may provide an additional route to understanding what happened.
The simulations came with substantial practical challenges. Running them took more than six months, and the datasets amounted to approximately 300 terabytes in total, requiring updates to older algorithms designed for smaller data volumes. Most of the computational work was carried out on the COSMA 8 supercomputer, hosted by Durham University's Institute for Computational Cosmology on behalf of the UK's DiRAC High Performance Computing Facility.
Future work with the simulation suite will target open questions such as where the very first generation of stars formed in the Universe can be found, and what the properties of ultra-faint dwarf galaxies reveal about the nature of dark matter.
Research Report:LYRA ultra-faints: The emergence of faint dwarf galaxies in the presence of an early Lyman-Werner background
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