The most discreet dwarf galaxies in our Universe reveal a gravitational anomaly that has intrigued astronomers for decades. While large spiral galaxies seem to follow well-established rules, the smallest ones contradict our expectations by rotating faster than predicted, suggesting the presence of an invisible component that escapes our telescopes.
An international team led by the Leibniz Institute for Astrophysics in Potsdam conducted an in-depth study of twelve of the smallest and faintest galaxies ever observed. By analyzing the speed of stars at different distances from the galactic center, researchers were able to map the internal gravitational field of these systems with unprecedented precision. The collected data clearly show that visible matter alone is insufficient to explain the intensity of the forces at play, challenging certain alternative theories.
Comparison between spiral galaxy M33 (left) and dwarf galaxy Eridanus II (right) showing differences in gravitational acceleration.
Credit: ESO/DSS2 (D. De Martin); DES (S.E. Koposov), composition: AIP (M. P. Júlio) The MOND theory (Modified Newtonian Dynamics), proposed in the 1980s as an alternative to dark matter, predicts that the laws of gravity change at very low accelerations. However, simulations performed on the British national supercomputer DiRAC demonstrate that this approach fails to reproduce the observed behavior in dwarf galaxies. In contrast, models incorporating a massive dark matter halo around these galaxies match the experimental data much better.
Mariana Júlio, a PhD student at the Leibniz Institute and lead author of the study, emphasizes that for the first time, scientists have been able to resolve the gravitational acceleration of stars in the faintest galaxies at different radii. This detailed analysis of internal dynamics confirms that the gravitational field cannot be determined solely by visible matter, thereby contradicting the predictions of modified gravity theories. These results significantly reinforce the need to invoke dark matter.
The research, accepted for publication in
Astronomy & Astrophysics and available on the
arXiv preprint server, also challenges the radial acceleration relation, a long-standing hypothesis that there exists a simple link between the amount of visible matter and the gravitational force produced. While this relationship remains valid for larger systems, it begins to break down in the smallest galaxies, where the same amount of visible matter can produce different gravitational accelerations.
Professor Justin Read from the University of Surrey explains that new modeling techniques now allow mapping the gravitational field at smaller scales than ever before. These advances offer new perspectives on this strange and invisible substance that constitutes the majority of the Universe's mass. Although these discoveries do not reveal the fundamental nature of dark matter, they significantly reduce the available space for alternative explanations.
Dark matter: the invisible cosmic enigma
Dark matter represents one of the greatest enigmas of modern cosmology. Scientists estimate that it constitutes about 85% of the total matter in the Universe, but it does not interact with light, making it impossible to observe directly with traditional telescopes.
Its presence is inferred indirectly through its gravitational effects on visible matter. Galaxies rotate so rapidly that without this additional invisible mass, they would disintegrate due to centrifugal force. Similarly, the bending of light by galaxy clusters, a phenomenon called gravitational lensing, reveals the presence of masses much greater than those that can be detected.
Current research focuses on several potential candidates, including WIMPs (Weakly Interacting Massive Particles), hypothetical particles that would interact only very weakly with ordinary matter. Other theories explore the possibility of axions, ultralight particles, or even primordial black holes formed shortly after the Big Bang.
The direct detection of dark matter remains a major goal of particle physics, with underground experiments like XENONnT in Italy or LZ in the United States seeking to capture the rare interactions between these mysterious particles and ordinary matter.
Galactic dynamics and its mysteries
Galactic dynamics studies the motion of stars and gas within galaxies, revealing crucial information about their structure and evolution. The orbital speeds of stars around the galactic center follow specific laws that depend on the total mass distribution, including both visible and invisible matter.
In the 1970s, astronomer Vera Rubin observed that stars in the outer regions of spiral galaxies moved at constant speeds, regardless of their distance from the center. This surprising discovery contradicted the predictions of Newtonian mechanics based solely on visible matter, providing the first solid evidence for the existence of dark matter.
Dwarf galaxies, like those studied in this research, exhibit particular dynamic characteristics. Their low luminosity and small size make them ideal laboratories for testing gravitational theories at extreme limits, where the effects of dark matter should be more pronounced.
Modern numerical simulations allow modeling the evolution of galaxies over billions of years, incorporating both baryonic (ordinary) matter and dark matter. These models help understand how dark matter halos influence the formation and evolution of cosmic structures, from the first galaxies to the galaxy clusters observable today.