Ghostly Trail Of Stars Reveals A New Way To Hunt Hidden Dark Matter
For the first time, astronomers have discovered a globular cluster stellar stream outside the Milky Way, giving scientists a new way to map invisible dark matter in distant galaxies.
Located approximately 115 million light-years from Earth, the faint ribbon of stars was spotted unraveling around UGC 9050-Dw1, an ultra-diffuse galaxy. Ultra-diffuse galaxies contain relatively few stars spread across a vast volume of space, creating a nice, dark backdrop. And that background proved crucial: it allowed the faint, delicate stellar stream to stand out in archival images captured by the Hubble telescope.
The finding, detailed in the Nature journal, was co-led by Julie Kiel Holm of the University of Copenhagen and Sarah Pearson of the Technical University of Denmark, alongside researchers from Northwestern University and the University of Arizona.
Stellar streams form when the gravitational pull of a host galaxy tears apart a globular cluster, which is a tight sphere of up to millions of stars. Rather than scattering randomly into space, the escaped stars remain trapped in a narrow trail, orbiting along nearly identical paths. Because every star in the stream acts as a tracker of gravitational forces, these cosmic trails preserve a record of the unseen mass influencing their motion.
Dark matter accounts for roughly 85% of all matter in the universe, yet because it neither absorbs, reflects, nor emits light, it cannot be observed directly—the only way around that is to rely on its gravitational imprint on visible objects to map its presence. The analysis confirmed that UGC 9050-Dw1 is shrouded in a heavy dark matter halo, thus validating both theoretical expectations and demonstrating that stellar streams can serve as gravitational scales for distant galaxies.
Consequently, extragalactic stellar streams offer a novel way to test how dark matter behaves on smaller scales. As concentrations of dark matter pass near a thin stellar stream, their gravitational pull can create visible gaps, kinks, or ripples in the stellar ribbon.
