Astronomers have identified the first stellar stream from a disintegrating globular cluster ever observed outside the Milky Way, a faint trail of stars that gives researchers a new way to measure dark matter in a galaxy other than our own. The stream was found threading through UGC 9050-Dw1, an ultra-diffuse dwarf galaxy roughly 115 million light-years away, in a study published in Nature.
The team, led by Julie Kiel Holm of the Niels Bohr Institute and Sarah Pearson of the Technical University of Denmark's DTU Space, spotted the stream in archival Hubble Space Telescope images taken with its Advanced Camera for Surveys. Because UGC 9050-Dw1 is unusually sparse in stars for its size, the faint, narrow ribbon — stars shed over billions of years as an ancient globular cluster slowly comes apart under the galaxy's gravity — stood out clearly against the dim background, something that would be far harder to detect in a more ordinary, star-dense galaxy.
Reading gravity in a trail of stars
Streams like this have been mapped for decades within the Milky Way, where they trace the shape of the galaxy's dark matter halo — the invisible scaffolding, accounting for roughly 85% of the universe's matter, that neither emits nor reflects light and can only be inferred from its gravitational pull. Confirming one beyond our own galaxy, as reported by ScienceDaily and Northwestern University, means the technique can now be tested on other kinds of galaxies, not only ones shaped like the Milky Way.
"We have discovered a globular cluster stellar stream in another galaxy. This is the first time such a stream has been observed outside our own galaxy."
Julie Kiel Holm, Niels Bohr Institute
To translate the stream's shape into a measurement, the researchers ran thousands of computer simulations, testing which combinations of the cluster's orbit and the galaxy's dark matter distribution could reproduce the stream's observed curvature and length. Co-author Tjitske Starkenburg of Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics said the approach lets the team work backward from the visible stars to the invisible mass around them. "By modeling that gravity, we can estimate the galaxy's total mass," she said.
Pearson said the result "opens entirely new possibilities" for the field, since the same method could now be applied to other ultra-diffuse galaxies. Researchers expect many more candidate streams once the Euclid Space Telescope's survey data matures and NASA's Nancy Grace Roman Space Telescope — expected to image roughly 100 times the sky area Hubble can in a single exposure — begins operating, giving astronomers a much larger sample of galaxies in which to hunt for similar faint trails and map dark matter beyond the Milky Way's own neighborhood.