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PC: W. M. Keck Observatory
For decades, dark matter has been treated as the unseen framework around which galaxies take shape. Even the smallest galaxies are generally thought to be wrapped inside vast halos of it, with the invisible material outweighing the stars by a wide margin.
That assumption has been challenged before by two faint galaxies known as DF2 and DF4, both of which appeared to contain far less dark matter than expected. Now a third object has joined that unusual group. Named DF9, the galaxy seems to be missing dark matter as well. More intriguingly, it lies along the same narrow chain of galaxies as the other two, strengthening the idea that all three were born from a single disruptive event that separated ordinary matter from its dark counterpart.
How scientists discovered the dark matter-deficient galaxy DF9
According to the study published in The Astrophysical Journal, titled “A Third Galaxy Missing Dark Matter along a Trail of Galaxies in the NGC 1052 Field”, using observations from the W. M. Keck Observatory in Hawaiʻi, the team examined DF9, a faint diffuse galaxy located in the NGC 1052 region. Their measurements suggest that the galaxy's mass can be explained almost entirely by the stars it contains, leaving little room for the dark matter that should ordinarily dominate a system of its size.
What makes DF9 especially interesting is its position. It is not an isolated oddity. The galaxy sits on the same remarkably straight trail of low-luminosity galaxies that already includes DF2 and DF4. Earlier work had shown that galaxies along this trail also share a coherent pattern of motion, hinting that they may have originated together rather than forming independently over billions of years. According to the research team, the discovery adds weight to the idea that the entire structure emerged from a common event rather than chance alignment.
How astronomers measured DF9's missing dark matter
Dark matter cannot be observed directly, so astronomers infer its presence through gravity. A galaxy containing large amounts of dark matter should show stars moving faster under the influence of that hidden mass.To investigate DF9, researchers used the Keck Cosmic Web Imager (KCWI) on the Keck II telescope. By analysing subtle shifts in starlight, they measured how rapidly stars move within the galaxy. The result was unexpectedly low.The team calculated that DF9 contains roughly 100 million times the mass of the Sun, closely matching the amount expected from its visible stars alone. If DF9 possessed a typical dark matter halo, its total mass should have been around one hundred times larger.As per the W. M. Keck Observatory, almost every known galaxy is dominated by dark matter, while DF2, DF4 and now DF9 appear to be rare exceptions. He added that the findings provide some of the strongest evidence so far that the three galaxies were created together during a violent event that separated ordinary matter from dark matter.
How a high-speed collision may have created dark matter-free galaxies
The idea receiving the most attention is sometimes described as a "bullet dwarf" scenario. In this picture, two gas-rich dwarf galaxies collided at extremely high speed. During the impact, ordinary gas could have been stripped away from the surrounding dark matter halo.That displaced gas may then have condensed into new galaxies containing stars but very little dark matter. Computer simulations had already suggested that such collisions could produce elongated chains of galaxies similar to the one observed around NGC 1052.The newly measured properties of DF9 fit that prediction surprisingly well. Before the galaxy was studied in detail, the collision model had anticipated that other members of the trail should also show a lack of dark matter if they formed from the same separated gas cloud.As per the study, the result provides strong evidence that dark matter behaves as a real physical substance rather than being merely an effect produced by alternative theories of gravity.
He noted that dwarf galaxies are among the most important places where such competing ideas are tested.
Astronomers continue to investigate the mystery of DF9
Astronomers remain cautious. The NGC 1052 trail is unlike anything previously observed, and questions remain about exactly how the system formed. Alternative explanations involving tidal interactions or material flung out during galaxy encounters have also been discussed.Still, the discovery of a third dark-matter-deficient galaxy changes the picture.
A single unusual galaxy might be dismissed as an anomaly. Two raised eyebrows. Three connected objects sharing the same structure and behaviour point toward a broader physical process.Future observations will focus on searching for leftover gas associated with the ancient collision and examining the remaining galaxies along the trail. Many are much fainter than DF9, making them difficult targets, but they could reveal whether the entire chain shares the same unusual characteristics.For now, DF9 stands as the latest piece of evidence that galaxies can sometimes form in ways astronomers rarely, if ever, expected. In a universe where dark matter usually appears inseparable from galaxy formation, this peculiar line of galaxies suggests that under the right circumstances, the two can be pulled apart.
