Fornax’s Ghost Twins: The Second-Ever Pair of Galaxies That Appear to Lack Dark Matter

The Case for Missing Dark Matter

What makes these two galaxies extraordinary is the mismatch between their visible stellar mass and their total dynamical mass. The research team measured extremely low velocity dispersions—just a few kilometers per second for their stars and globular clusters . Using a standard mass estimator, the inner dynamics of both galaxies are more naturally explained by stars alone than by any plausible dark-matter halo, whether cuspy or cored . In short, the dynamical mass appears to be what you see in stars, with almost nothing left over for dark matter.

Stellar Populations and Globular Clusters

Both FCC 224 and FCC 240 are ancient systems. Spectroscopy and multiwavelength photometry show a mass-weighted age of approximately 10 billion years, a low metallicity around –1.25 dex, and an extremely short formation e-folding time of roughly 0.3 billion years, consistent with a single violent starburst . FCC 224 hosts at least 12 candidate globular clusters that are unusually luminous and homogeneous in color, with very small sizes of ~3 parsecs and evidence of radial mass segregation—another hallmark of an unusual formation history . FCC 240’s globular cluster system appears similarly overluminous, although less extensively catalogued in the initial discovery paper .

A Comparison to NGC 1052-DF2 and DF4

The Fornax pair serves as a near-perfect analogue of the iconic NGC 1052 group galaxies DF2 and DF4. Both pairs share the same three unusual traits: low velocity dispersions, overluminous globular clusters, and a severe deficit of dark matter . For more than five years, DF2 and DF4 stood alone as the only confirmed candidate pair. FCC 224 and FCC 240 are now the second such pair ever identified .

Key environmental differences exist, however. DF2 and DF4 belong to the NGC 1052 group and form part of a linear “trail” of roughly 11 dwarf galaxies. FCC 224 and FCC 240 reside in a richer cluster environment and are more compact, with a projected separation of roughly 10 kpc, compared with the wider DF2–DF4 separation . FCC 224 was first flagged as a dark-matter–deficient candidate in 2024–2025 from its overluminous star clusters; the 2026 VLT/MUSE campaign confirmed its dark-matter deficiency and revealed FCC 240 as a companion .

How Bullet-Dwarf Collisions Forge Dark-Matter–Free Galaxies

The leading theoretical explanation for these ghost galaxies is the “bullet-dwarf” (or “mini-bullet”) collision scenario. The model proposes a high-velocity, head-on collision between two gas-rich dwarf galaxies on scales hundreds to thousands of times smaller than the famous Bullet Cluster .

During such a collision, the gas clouds—unlike the dark-matter halos and stars—collide, shock, and decouple. The collisionless dark-matter halos pass through one another and continue on, while the stripped, compressed gas cools and fragments into new stellar systems. These newborn dwarfs inherit stars and newly formed globular clusters from the shocked gas but almost none of the original dark matter .

The bullet-dwarf model makes testable predictions that align well with the observed systems:

• Overluminous globular clusters, born from dense, shocked gas .
• Nearly identical globular-cluster colors and stellar ages within each pair, reflecting simultaneous formation .
• Linear alignments of multiple dark-matter–deficient dwarfs along the collision axis, a “beads-on-a-string” morphology that has been observed in the NGC 1052 group and simulated successfully .

For FCC 224 and FCC 240, the close separation, shared ancient age, and rapid formation timescale are all naturally explained by a single collision event 8–10 billion years ago .

Cosmological Stakes: Contradiction or Prediction?

In the standard ΛCDM framework, every galaxy should sit inside a dark-matter halo proportional to its stellar mass. Galaxies whose dynamical mass equals their stellar mass appear to contradict that expectation directly . However, the bullet-dwarf scenario reframes this paradox. Rather than falsifying ΛCDM, these objects might be a rare but predictable outcome of galaxy collisions that separate gas from dark matter. Simulations of satellite–satellite collisions outside the host-galaxy virial radius have already reproduced the observed UDGs in the NGC 1052 group . If future simulations confirm that the same mechanism works in the Fornax environment, dark-matter–deficient galaxies would switch from a contradiction to a prediction of the standard model .

The existence of two independent pairs in different cosmic environments makes it far less likely that these are measurement errors or isolated flukes. The data suggest a genuine class of objects . Moreover, the recent addition of a third dark-matter–deficient galaxy, NGC 1052-DF9, along the same trail as DF2 and DF4 has further bolstered the collision scenario .

Open Questions and Alternative Explanations

Despite the appeal of the bullet-dwarf hypothesis, key uncertainties remain, particularly for the Fornax pair. The model has been tested explicitly through simulations for the NGC 1052 group but not yet for FCC 224 and FCC 240 . An alternative explanation—tidal stripping of dark matter by the Fornax Cluster’s gravitational potential—has not been ruled out . Distinguishing between collision-born galaxies and tidally stripped dwarfs will require deeper dynamical modeling and, ideally, the discovery of a linear trail of additional dark-matter–deficient galaxies around FCC 224 and FCC 240.

What is clear is that the hunt is on. FCC 224 and FCC 240 have opened a second window into the violent, dark-matter–separating collisions that may sculpt the universe’s most ghostly galaxies.