Dark star clusters or extreme dwarf galaxies? Astrophysicists revisit Ursa Major III’s true nature

Ursa Major III, the faintest object in our galaxy, orbits the Milky Way at a distance of more than 30,000 light years. Until now, it was considered a dwarf galaxy, thought to consist mainly of dark matter due to its large mass. However, an international team of astrophysicists from the University of Bonn and the Institute for Advanced Studies in Basic Sciences in Iran has found evidence suggesting that it is actually a compact star cluster containing a black hole core. The study has been published in The Astrophysical Journal Letters.

The study focuses on celestial bodies that cannot yet be clearly categorized as either star clusters or dwarf galaxies. While they outwardly resemble classic star clusters, they have unusually high mass-to-light ratios; some are hundreds to thousands of times higher than those of typical dwarf galaxies. This peculiarity has led to the assumption that they contain large amounts of dark matter.

“Neither established dark matter models nor alternative theories have been able to satisfactorily explain the exact causes. Such intermediate objects are therefore considered a ‘hot topic’ in astrophysics and are the subject of intensive research,” says doctoral student and first author Ali Rostami-Shirazi from the Iranian Institute for Advanced Studies in Basic Sciences.

Focus on Ursa Major III: New evidence of a dark star cluster

Ursa Major III is the faintest known satellite of the Milky Way. These small companion galaxies orbit the Milky Way and provide important clues about its formation and composition. Previously considered a dark dwarf galaxy—a small galaxy whose mass is thought to consist mainly of dark matter—Ursa Major III has now been found to be a dark star cluster. Simulations by the research team now suggest that Ursa Major III could be a compact star cluster whose gravity is held together by a core of black holes and neutron stars rather than dark matter.

“Dark star clusters form when gravitational interactions with the Milky Way over billions of years remove the outer stars from a star cluster,” explains Prof. Dr. Hosein Haghi, who is conducting research at the University of Bonn and is affiliated with the Iranian Institute for Advanced Studies in Basic Sciences in Zanjan. What remains is a dark, massive core that does not emit any light. According to the study, this effect has previously been mistakenly interpreted as evidence of dark matter.

Testing the simulations

To test the hypothesis, the research team simulated the evolution of Ursa Major III over cosmic timescales. Using specialized N-body simulations, which calculate the gravitational interactions of thousands of stars with great precision, the team reconstructed the development of Ursa Major III’s current structure over time. These simulations are based on the latest observational data, including Ursa Major III’s orbital motion and chemical composition.

The research team’s calculations show that the observed state of Ursa Major III can be explained by a dense core of black holes holding the remaining stars together gravitationally, without the need for dark matter.

“Our work shows for the first time that these objects are most likely normal star clusters,” says Prof. Dr. Pavel Kroupa, who is a member of the Transdisciplinary Research Areas (TRA) Modeling and Matter at the University of Bonn. “These results solve a major mystery in astrophysics.”

Such problems can be solved effectively with the right approach to computer simulations, causing seemingly “exotic components” in astrophysics to disappear.

The Bonn team considers itself a leader in this field. Over many years, they have developed specialized numerical methods to map the highly complex dynamics of such star systems in detail. Kroupa says, “Our current results provide a new basis for understanding mysterious celestial objects, while also opening up new perspectives for galaxy research.”