Two dark matter deficient galaxies discovered in Fornax cluster
Astronomers have identified two unusual galaxies in the Fornax cluster that appear to contain almost no dark matter. The objects, FCC 224 and FCC 240, sit on the outskirts of the cluster at roughly 60 million light years from Earth. Their internal dynamics suggest that their visible stars alone can explain their gravitational behaviour, challenging standard models of galaxy formation that predict dominant dark matter halos.
Observations were carried out using high resolution spectroscopy from a major southern hemisphere telescope. Researchers measured the motion of stars and globular clusters inside both galaxies. The results showed extremely low velocity dispersion, meaning the stars move too slowly to indicate the presence of the large unseen mass normally expected. In both systems, dynamical mass closely matches stellar mass, a pattern rarely observed in dwarf galaxies.
The two galaxies also host unusually bright globular clusters with similar luminosity distributions and chemical composition. These properties closely match those seen in two previously identified dark matter deficient galaxies, suggesting a possible shared formation mechanism. One leading hypothesis proposes a high speed collision between gas rich dwarf galaxies, where dark matter halos pass through each other while ordinary matter interacts and forms new stars.
FCC 224 and FCC 240 strengthen this scenario but also introduce new questions. They share nearly identical ages of about 10 billion years, and their star clusters appear to have formed in the same event. However, unlike earlier cases that are widely separated and connected by long streams of debris, these two galaxies are only about 75 kiloparsecs apart and are moving toward each other at a slow relative speed. This compact pairing suggests a gravitationally bound system that may represent a different stage of evolution or a more common pathway for dark matter loss than previously assumed.
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