Possible dark matter-deficient twins discovered in the Fornax Cluster

June 9, 2026 report Possible dark matter-deficient twins discovered in the Fornax Cluster Shreejaya Karantha Author Gaby Clark Scientific Editor Robert Egan Associate Editor Astronomers have identified a possible new example of one of the universe's strangest galaxy types: galaxies that appear to contain little or no dark matter. The newly studied pair, FCC 224 and FCC 240, on the outskirts of the Fornax Cluster, share several unusual traits with the only known pair of controversial dark-matter-deficient galaxies. The findings were uploaded to the arXiv preprint server on May 22. Ghost galaxies Ultra-diffuse galaxies are faint systems that are roughly the size of the Milky Way but have much less mass, containing far fewer stars. They have sparked debate for more than 10 years, mainly because they have been observed with two contrasting levels of dark matter content. On one end, the dark-matter-rich ultra-diffuse galaxies are reasonably well understood: These are thought to be "failed galaxies" quenched early, never building much stellar mass but holding on to many globular clusters. The opposite extreme is far stranger. A small number of ultra-diffuse galaxies appear to contain little or no dark matter at all, and the globular clusters they host are unusually bright. Researchers think that the same process responsible for the clusters' overluminosity could also be responsible for the absence of dark matter in these rare systems. So far, only two such galaxies have been found: NGC 1052-DF2 and NGC 1052-DF4, both associated with the NGC 1052 group. This raises the question of whether this is a unique local oddity rather than a general phenomenon. Recently, a third dark-matter-deficient dwarf galaxy, FCC 224, was found outside the NGC 1052 group. In the new study, a team of astronomers led by Maria Luísa Buzzo of Yale University studies FCC 224 and a nearby companion galaxy, FCC 240, in the Fornax Cluster using the MUSE instrument on the Very Large Telescope to explore whether these two galaxies form a pair analogous to DF2 and DF4. The team also tests the "bullet-dwarf" scenario as their origin story. It proposes that when two dwarf galaxies collide at high speed, the violent collision can separate stars from much of their dark matter. Connecting the dots Here is what they found: Both galaxies have extremely low velocity dispersions. Stars and globular clusters are moving so sluggishly that the inner gravitational potential is more naturally explained by the stars alone than by any dark matter halo. Their dynamical masses within the half-light radius are consistent with their stellar masses alone, far below the values expected for typical dwarf galaxies. Both also host abnormally bright globular clusters with a top-heavy luminosity function closely matching DF2 and DF4. Additionally, FCC 224 and FCC 240 are almost exactly the same age, at around 10 billion years old. Their globular clusters share the same age and metallicity as the diffuse stellar bodies. That means stars and clusters formed together in a single intense burst. According to a study published in the Astrophysical Journal Letters, this is one of the strongest predictions of the bullet-dwarf scenario. The collision triggers a short, violent episode of star formation that produces both the galaxy's diffuse stellar body and its unusually massive clusters at the same time. However, these galaxies show one difference from the DF2 and DF4 pair. The NGC 1052 pair are widely separated by a distance of about 240 kiloparsecs and are associated with an extended trail of low-surface-brightness galaxies stretching more than two megaparsecs, the debris of a past collision. FCC 224 and FCC 240 are far closer together, separated by only 75 kiloparsecs, and moving toward each other at just 16 kilometers per second. They appear to form a compact, long-lived bound pair rather than a stretched-out trail of fragments. Making all the difference This difference suggests that it is possible the two galaxies formed independently and happen to share similar properties by coincidence—"a possibility that can only be tested with deeper and more comprehensive data," they write in the paper. The team also estimated that any dark matter remnants from the original collision should lie roughly three megaparsecs away along the projected axis—a natural place to search for additional fragments or the ejected dark matter halo. The fragments along the trajectory may not have been identified yet. Nevertheless, they argue that this may still represent another example of the same phenomenon, starting with a slight variation in the geometry or orbital energy of the encounter and thereby creating a tighter pair instead of an extended chain of fragments. "Taken together, these results suggest that FCC 224 and FCC 240 may have formed in a high-velocity collision similar to the event proposed for DF2 and DF4," they conclude. Overall, the process that created DF2 and DF4 may not be unique to the NGC 1052 group. Written for you by our author Shreejaya Karantha, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you. Publication details Maria Luisa Buzzo et al, Dark matter-deficient twins: FCC 224 and FCC 240 as possible analogues of NGC 1052-DF2 and DF4, arXiv (2026). DOI: 10.48550/arxiv.2605.24099 Journal information: Astrophysical Journal Letters , arXiv © 2026 Science X Network