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Astronomers using W. M. Keck Observatory on Maunakea, Hawaiʻi Island have found compelling evidence of a dark matter-deficient dwarf galaxy, FCC 224. This ultra-diffuse galaxy is located on the outskirts of the Fornax Cluster, approximately 60–65 million light-years from Earth.

The discovery challenges the traditional cosmological paradigm, which assumes dark matter is a fundamental component in galaxy formation. Previously, dark matter-deficient galaxies without any ongoing star formation had only been claimed in one other place, the NGC 1052 group, and the new observations suggest such objects may be more widespread than previously thought.

The unusual galaxy was profiled in two complementary studies. The first study, led by Doctoral Candidate Yimeng Tang of the University of California, Santa Cruz and in the Astrophysical Journal, analyzes FCC 224's unique globular star cluster system.

The second, led by Doctoral Candidate Maria Luísa Buzzo of Swinburne University and of the European Southern Observatory, and in Astronomy & Astrophysics, explores the dark matter content of the galaxy and possible scenarios for its formation.

"Globular clusters are often used as a proxy to estimate the amount of dark matter a galaxy contains," said Buzzo. "For some reason this galaxy (FCC 224) has an unusual number of luminous clusters and no dark matter, at least within its inner regions. No existing galaxy formation model within our standard cosmological paradigm can currently explain how this galaxy came to be."

Buzzo's research investigated the dark matter content of FCC 224 using high-resolution spectroscopy from Keck Observatory's Keck Cosmic Web Imager (KCWI). She explored its similarities to previously claimed dark matter-free dwarf galaxies DF2 and DF4, located within the NGC 1052 galaxy group, to determine whether or not FCC 224 belongs to the same class of galaxies.

Buzzo's research builds upon the initial findings of Tang, who, using imaging from NASA's Hubble Space Telescope (HST), found that FCC 224 has star clusters similar to other dark matter-deficient galaxies. Their research suggests that these types of star clusters may be linked to a lack of dark matter, challenging conventional models of galaxy formation. The Keck Observatory/KCWI data was obtained through a Swinburne University proposal under Principal Investigator (PI) Duncan Forbes, while the HST data was acquired through a program led by PI Aaron Romanowsky.

"The new high-resolution red arm of KCWI enabled us to measure very precisely the motions within the galaxy, which can be used to trace its dark matter content," explained Buzzo. "KCWI's high spectral resolution allowed us to precisely measure the motions (velocity dispersion) within FCC 224's stars and globular clusters," said Tang. "These measurements confirmed that the galaxy exhibits an extremely low velocity dispersion—a hallmark of dark matter deficiency."

Both teams highlight the need for further observations to understand the mechanisms behind FCC 224's formation. One leading hypothesis is that an external interaction, such as a high-velocity collision, may have separated the galaxy's dark and visible matter, a formation scenario known as 'the bullet-dwarf model." However, additional data is required to test this theory.

"FCC 224 serves as a crucial data point in our effort to identify and study other dark matter-deficient galaxies," Buzzo added. "By expanding the sample size, we can refine our understanding of these rare galaxies and of the role of dark matter in dwarf galaxy formation."

The discovery of FCC 224 outside the NGC 1052 group suggests that dark matter-deficient galaxies may be more common than previously believed, raising fundamental questions about the necessity of dark matter in shaping galaxies. As observational capabilities improve, astronomers aim to conduct larger surveys to identify similar galaxies and investigate their origins.