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The above image might not look unusual, but it poses an interesting mystery for astronomers. Captured by the Hubble Space Telescope, it is an image of an ultra-diffuse galaxy (UDG) known as FCC 224. Unlike regular dwarf galaxies such as the Magellanic Clouds, these galaxies have an unusually low surface brightness for a galaxy of their size. This usually means they contain lots of dark matter. The gravity of the dark matter holds the galaxy together rather than the stars and cosmic dust.<\/p>\n
The standard cosmological model predicts UDGs. Since dark matter is more common than regular matter, it’s the clumping of dark matter that drives galaxy formation. Sometimes a dark matter clump can form without a lot of stellar material around, and you get a galaxy without lots of stars. But in recent years, astronomers have found that some diffuse galaxies aren’t dominated by dark matter. The evidence pointing to a lack of dark matter wasn’t definitive, but it was strong enough for astronomers to wonder about how such a galaxy could form. One idea is that they are a result of a recent collision that stripped the diffuse galaxy of its dark matter. Other astronomers argue that such a galaxy would support alternative models, such as modified gravity.<\/p>\n
This new galaxy has two things going for it. The first is that FCC 224 is not part of a galactic cluster known as the NGC 1052 group. This means we can just argue that these odd UDGs are the result of some unique property of a particular cluster. In one study, the authors show that there are shared characteristics among these galaxies, such as having old, metal-poor stars, low levels of dust, and a slow rate of rotation. This suggests that dark matter free ultra-diffuse galaxies can be categorized as a specific type of galaxy.<\/p>\n
A second study looks at globular clusters with FCC 224. They are unusually small and tightly packed, which would throw a wrench in the collision model. If a collision was strong enough to strip the galaxy of dark matter, it should also have been strong enough to disrupt the globular clusters.<\/p>\n
Overall, these studies confirm the existence of ultra-diffuse galaxies without dark matter, and their existence suggests we may have to revise the standard model. <\/p>\n
Reference:<\/strong> Buzzo, Maria Luisa, et al. “A new class of dark matter-free dwarf galaxies? I. Clues from FCC 224, NGC 1052-DF2 and NGC 1052-DF4.” arXiv preprint<\/em> arXiv:2502.05405 (2025).<\/p>\n Reference:<\/strong> Tang, Yimeng, et al. “An Unexplained Origin for the Unusual Globular Cluster System in the Ultra-diffuse Galaxy FCC 224.” arXiv preprint<\/em> arXiv:2501.10665 (2025).<\/p>\n<\/p><\/div>\n