Astronomers studying a distant ultra-diffuse galaxy 115 million light-years away have identified a ghostly ribbon of stars that could help map elusive dark matter.
Discovering a Ghostly Ribbon of Stars 115 Million Light-Years Away
A thin, winding structure surrounds UGC 9050-Dw1, an ultra-diffuse galaxy located 115 million light-years from Earth. Researchers identified a globular cluster stellar stream in another galaxy, extending a powerful technique for studying gravity and dark matter far beyond our own galactic neighborhood.
The finding was reported in Nature by an international team including Northwestern University astrophysicist Tjitske Starkenburg. Julie Kiel Holm of the University of Copenhagen and Sarah Pearson of the Technical University of Denmark co-led the research. David Sand and Catherine Fielder, both astronomers at the University of Arizona, had worked with archival observations from NASA’s Hubble Space Telescope, while study coauthor David Hendel noticed the narrow, curved feature while examining published images of the galaxy.
How Globular Clusters Stretch Into Stellar Streams
Globular clusters are compact, gravitationally bound groups containing large numbers of stars. However, they are not immune to the gravity of the galaxies they orbit. Over time, a host galaxy can pull stars away from a cluster. Rather than scattering in every direction, those escaped stars tend to remain close to the cluster’s orbit, gradually stretching into narrow leading and trailing streams that can survive for billions of years.
That orderly motion makes stellar streams scientifically useful. Because their trajectories respond to a galaxy’s gravitational field, their shapes can reveal how mass is distributed, including mass that cannot be seen directly.
Using Visible Tracers to Map Invisible Mass
Dark matter accounts for roughly 85% of the universe’s matter, yet it does not emit, absorb, or reflect light in a way that conventional telescopes can directly detect. Its presence is instead inferred from its gravitational influence on stars, galaxies, and larger cosmic structures. Ultra-diffuse galaxies like UGC 9050-Dw1 contain relatively few visible stars spread across a large area, creating a dim background against which an extremely faint stream can stand out.
Tjitske Starkenburg of Northwestern University explained that by modeling that gravity, they could estimate the galaxy’s total mass, noting that since they already knew roughly how much of that mass came from visible matter like stars, the rest had to be dark matter.
Earlier research using Hubble and radio observations found that UGC 9050-Dw1 has a distorted appearance, a prominent stellar plume, and an unusually rich population of globular clusters. Researchers estimated that it contains about 52 globular clusters, contributing roughly 20% of the galaxy’s total light. After identifying the structure, the researchers generated thousands of computer simulations, varying the properties of the original globular cluster and the distribution of matter throughout the galaxy, to confirm that the stream originated from a globular cluster.
Early Stars and Supermassive Dark Stars
In a separate theoretical development reported in Physical Review D, physicists Sohan Ghodla and Cosmin Ilie of Colgate University examined the hypothesis of supermassive dark stars. The concept of a Dark Star was originally proposed in 2007 by Katherine Freese and Paolo Gondolo of the University of Utah. Their reasoning was based on the observation that dark matter dominates visible matter by mass and gravity, and was significantly denser during the first few hundred million years of the observable Universe.
According to Futura Sciences coverage of the research, the first stars born inside dark matter halos would have accreted significant quantities of dark matter during their formation, providing researchers with theoretical models of stars that could shine by burning dark matter rather than traditional nuclear fusion.
