Ghostly Star Stream Beyond the Milky Way Reveals Hidden Dark Matter

An international team of astronomers has found the first stellar stream from a globular cluster ever seen outside the Milky Way, in a dwarf galaxy called UGC 9050-Dw1, about 115 million light-years away. The find, published August 12 in the journal Nature, was co-led by Julie Kiel Holm of the University of Copenhagen and Sarah Pearson of the Technical University of Denmark, with Northwestern University's Tjitske Starkenburg among the coauthors.

A globular cluster is a dense group of stars bound by gravity. As it orbits its host galaxy, gravity strips stars away, and they trail along nearly the same orbit, forming a long, thin stream. Astronomers have found dozens of such streams inside the Milky Way, but none confirmed elsewhere — they are usually too faint to see. Coauthor David Hendel spotted a faint arc in old Hubble Space Telescope images from University of Arizona astronomers David Sand and Catherine Fielder; the galaxy has few stars of its own, giving the stream a dark backdrop.

Dark matter, about 85% of all matter in the universe, cannot be seen directly and is detected only through its gravitational pull. "The stars in a stream all travel along nearly the same orbit, shaped by the galaxy's gravity," Starkenburg said. "By modeling that gravity, we can estimate the galaxy's total mass; we know how much comes from visible matter like stars, so the rest must be dark matter." The team ran thousands of simulations testing cluster properties and dark matter distributions, and found it holds a large amount of dark matter, as expected for this type of diffuse galaxy.

Researchers say small clumps of dark matter passing through a thin stream can leave visible gaps, a feature long debated in Milky Way streams; confirming it here could reveal more about dark matter's nature. The European Space Agency's Euclid mission and NASA's Roman Space Telescope, which survey far larger sky areas than Hubble, are expected to find many more such streams.