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Discovery of Dark Matter-Deficient Galaxy FCC 224 Challenges Cosmological Norms

A team of astronomers from W. M. Keck Observatory, located on Maunakea, Hawaiʻi Island, has unveiled intriguing evidence of a dwarf galaxy known as FCC 224, notable for its lack of dark matter. This ultra-diffuse galaxy resides on the outskirts of the Fornax Cluster and is situated around 60 to 65 million light-years from our planet.

The find casts doubt on the established cosmological frameworks that posit dark matter as an essential building block in the formation of galaxies. Until now, the existence of galaxies deficient in dark matter, especially those without ongoing star formation, had been noted only in the NGC 1052 group. This new discovery suggests such dark matter-deficient galaxies could be more prevalent than previously recognized.

The investigations into FCC 224 took place across two complementary studies. The first study, spearheaded by Doctoral Candidate Yimeng Tang from the University of California, Santa Cruz and published in the Astrophysical Journal, delves into the galaxy’s distinctive system of globular star clusters.

Meanwhile, the second study was led by Doctoral Candidate Maria Luísa Buzzo from Swinburne University and the European Southern Observatory, which also found its publication in Astronomy & Astrophysics. This research investigates the dark matter characteristics of FCC 224 and explores various potential formation scenarios.

“Globular clusters can signal the dark matter density within a galaxy,” noted Buzzo. “In the case of FCC 224, its unusually high count of luminous clusters, coupled with the absence of dark matter in its inner regions, presents a mystery that our current galaxy formation models can’t adequately explain.”

Buzzo’s examination employed high-resolution spectroscopy using the Keck Cosmic Web Imager (KCWI) to analyze FCC 224’s dark matter content. She compared her findings to previously identified dwarf galaxies DF2 and DF4, also believed to be dark matter-free, located within the NGC 1052 group, in order to determine if FCC 224 fits into this emerging class of galaxies.

This research enhances Tang’s initial discoveries, who utilized imaging from NASA’s Hubble Space Telescope (HST) to find that FCC 224 possesses star clusters akin to other dark matter-deficient galaxies. Their collective findings propose that the properties of these star clusters might be associated with the observed lack of dark matter, challenging conventional galaxy formation theories. The observations from Keck Observatory were part of a project proposed by Principal Investigator Duncan Forbes, while the HST data was gathered under Principal Investigator Aaron Romanowsky.

“The advanced red arm of KCWI allowed us to obtain precise measurements of stellar motions within the galaxy, which can help us infer its dark matter content,” explained Buzzo. Tang added, “Our measurements indicated that FCC 224 has an exceptionally low velocity dispersion, which is indicative of dark matter deficiency.”

Further investigations are deemed essential by both teams to unveil the underlying processes that led to FCC 224’s current formation. A prominent hypothesis suggests that an external event, such as a high-velocity collision, could have resulted in a separation of the galaxy’s dark and visible matter—a concept referred to as the ‘bullet-dwarf model.’ However, further data is needed to validate this theory.

Buzzo remarked, “FCC 224 is a vital piece of evidence in our efforts to locate and examine additional dark matter-deficient galaxies. Expanding our sample size will help refine our understanding of these unique galaxies and the overall role of dark matter in dwarf galaxy evolution.”

The finding of FCC 224, independent of the NGC 1052 group, indicates that dark matter-deficient galaxies might be more common than once thought, prompting critical re-evaluations of the role of dark matter in the formation of galaxies. As observational techniques advance, astronomers are poised to undertake larger surveys to identify and analyze similar galaxies and unravel their development histories.

More information: Yimeng Tang et al., “An Unexplained Origin for the Unusual Globular Cluster System in the Ultradiffuse Galaxy FCC 224,” The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/adae11

Maria Luisa Buzzo et al., “A new class of dark matter-free dwarf galaxies?” Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202453522

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