TEHRAN -- An international team of astrophysicists, including researchers from Iran’s Institute for Advanced Studies in Basic Sciences (IASBS) and the University of Bonn, has revealed that Ursa Major III, long considered the faintest known dwarf galaxy orbiting the Milky Way, is not a dark matter-rich galaxy after all—but likely a compact star cluster with a black hole core.
The study, published in The Astrophysical Journal Letters, highlights the role of Iranian scientists in reshaping our understanding of borderline objects that fall between dwarf galaxies and star clusters.
“These objects exhibit unusually high mass-to-light ratios, some hundreds of times greater than typical galaxies. This had led to assumptions they were dominated by dark matter,” explains Ali Rostami-Shirazi, a doctoral researcher at IASBS and first author of the study. “Yet neither standard dark matter models nor alternative gravity theories have offered a satisfactory explanation—making these objects a hot topic in astrophysics.”
Ursa Major III, also known as UNIONS 1, consists of about 60 visible stars and is located over 30,000 light-years from Earth. Although previously classified as a dark dwarf galaxy, simulations by the team now suggest it is a remnant star cluster, gravitationally held together by a dense core of black holes and neutron stars, not dark matter.
Professor Hussein Haghi, a senior astrophysicist at IASBS and affiliated with the University of Bonn, emphasizes that gravitational interactions over billions of years may have stripped the cluster of its outer stars, leaving behind a dark, massive core mistaken for dark matter.
Using advanced N-body simulations and the latest observational data, including Ursa Major III’s orbit and chemical composition, the team traced the system’s evolution. Their models showed that the cluster’s current structure can be explained without invoking any dark matter.
“This is the first time such objects are shown to be normal star clusters rather than dwarf galaxies,” said Prof. Pavel Kroupa of the University of Bonn. The findings, made possible through close Iranian-German collaboration, not only challenge long-held theories but also offer a new framework for interpreting mysterious cosmic structures.