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Ghostly Galaxy Perplexes Astronomers with Lack of Dark Matter
Astronomers have discovered another enigmatic, virtually invisible galaxy seemingly without dark matter, the mysterious substance believed to constitute a significant portion of the universe. This ultradiffuse galaxy, named FCC 224, adds to a growing collection of celestial objects that challenge current cosmological models.
The Mystery of Dark Matter Deficient Galaxies
Dark matter is thought to provide the gravitational framework necessary for galaxies to form and evolve. The existence of a galaxy lacking this substance presents a significant puzzle, akin to observing a shadow without an object to cast it.
Over the past decade, scientists have reported several instances of such galaxies. Notably, these have predominantly been ultradiffuse galaxies, characterized by their considerable size – comparable to the Milky Way – but with a remarkably low density of stars.
FCC 224: A Newly Identified Dark Matter-Poor Galaxy
FCC 224, the latest addition to this intriguing group, is located on the outskirts of the Fornax Cluster, a cluster of galaxies approximately 65 million light-years from Earth.
Related: What is dark matter?
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Initially identified in 2024, FCC 224 is classified as a dwarf galaxy. It contains an unusually high number of bright, tightly packed star clusters for its size, a feature typically observed in larger galaxies rich in dark matter. However, evidence suggests it is deficient in this enigmatic substance. Furthermore, its location differs from previously discovered dark matter-lacking galaxies, implying these objects might not be anomalies but could represent a more widespread, previously unrecognized category of dwarf galaxies, according to findings published in two recent, related studies.
Challenging Galaxy Formation Models
“Current galaxy formation models within our established cosmological understanding are unable to explain the origins of this type of galaxy,” stated Maria Buzzo, a doctoral candidate in astrophysics at Swinburne University of Technology in Australia and lead author of one of the studies, in a statement.
Buzzo’s team utilized data from the Keck Observatory in Hawaii to measure the movement of star clusters within FCC 224. The observed slow velocity of these clusters indicates a weak gravitational pull, suggesting a lack of dark matter, as detailed in the study. Researchers emphasize that no known scenario fully accounts for FCC 224’s observed characteristics.
Possible Formation Scenarios
Another research team, led by astronomer Yimeng Tang at the University of California, Santa Cruz, compared FCC 224 to other galaxies seemingly lacking dark matter, particularly focusing on two ghostly objects within the NGC 1052 group, also roughly 65 million light-years away in the constellation Cetus.
Tang and colleagues propose a formation theory for FCC 224, similar to the NGC 1052 dwarf galaxies: a high-speed collision between gas-rich galaxies. In such an event, gas can become separated from dark matter, and subsequent star formation within the expelled gas results in the creation of galaxies devoid of dark matter.
Potential Twin Galaxy and Alternative Explanations
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Prior research suggests that the two galaxies DF2 and DF4 in the NGC 1052 group are part of a sequence of seven to eleven dark matter-deficient galaxies formed from the same ancient collision. Tang’s team suggests in their study that FCC 224 may also possess a twin galaxy.
FCC 240, a nearby galaxy exhibiting similar size, shape, and orientation to FCC 224, is considered a potential candidate. If future observations confirm shared characteristics, it would offer significant support for the collision scenario as the origin of FCC 224, according to the researchers.
The Significance of FCC 224
Alternatively, the team proposes that FCC 224 could have formed in a turbulent, high-energy environment where intense star formation from exceptionally massive star clusters ejected dark matter from the galaxy.
“FCC 224 is a vital piece of information in our quest to find and analyze more dark matter-deficient galaxies,” Buzzo stated. “Increasing the number of studied examples will allow us to refine our understanding of these unusual galaxies and the role of dark matter in the formation of dwarf galaxies.”