Third Dark-Matter-Free Galaxy Discovered in NGC 1052 Chain

NGC 1052-DF9, a dwarf galaxy 67 million light-years distant, exhibits motions explainable without dark matter. Its size, brightness, and globular-cluster system closely match DF2 and DF4, making it the third dark-matter-deficient galaxy in the same linear arrangement near NGC 1052.

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DF2 was reported in 2018 as having far less dark matter than expected; DF4 showed the same in 2019. In 2022 the team identified a linear chain that includes DF2 and DF4 and contains about a dozen dwarf galaxies. The 2025 kinematic study of DF9 confirmed that all three share similar velocities, indicating a common origin.

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Michael Keim (Yale) led the study with Pieter van Dokkum and collaborators. Milestones: 2018 DF2 paper, 2019 DF4 paper, 2022 identification of the galaxy chain, and 2025 DF9 kinematic analysis in *The Astrophysical Journal*.

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NGC 1052-DF9 is 67 million light-years away; the chain contains about a dozen dwarf galaxies. DF2, DF4, and DF9 have similar luminosities, sizes, globular-cluster populations, and identical line-of-sight velocities. Keim and van Dokkum suggest a ‘bullet dwarf collision’ where two dwarf galaxies collided head-on, so stars and dark-matter halos passed through while gas stalled and formed stars, leaving regions rich in ordinary matter but depleted of dark matter, creating galaxies like DF9.

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If dwarf galaxies can form without dark matter, the observations test dark-matter physics. The results favor models where dark matter behaves as a physical substance rather than an emergent effect of modified gravity, especially at the dwarf-galaxy scale where alternative theories are most contested.

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Keim called the three galaxies “extraordinary exceptions” to usual dark-matter dominance and said their alignment offers a “rare new window into the nature of dark matter.” Van Dokkum noted the finding “provides compelling evidence that dark matter behaves as a physical substance rather than the effect of an alternative theory of gravity.” No dissent appears, though the authors note uncertainty about the exact collision mechanism.

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• “A line of galaxies lacking dark matter has never been seen before,” — Michael Keim, Astrophysicist, Yale University

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• “ We don't know what that process was, but the researchers believe a type of event called a "bullet dwarf collision" may be the culprit.” — Michael Keim, Astrophysicist, Yale University

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• “provides compelling evidence that dark matter behaves as a physical substance rather than the effect of an alternative theory of gravity, particularly at the dwarf-galaxy scale where those theories are most heavily debated.” — Pieter van Dokkum, Astrophysicist, Yale University

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Keim and his collaborators will extend spectroscopic surveys of remaining NGC 1052 filament members, test the bullet-collision model, and simulate gas-driven star formation after collisions.

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