Ghostly River of Stars Around a Faint Galaxy Offers New Way to Trace Dark Matter

Astronomers have discovered a faint ribbon of stars stretching around a galaxy named UGC 9050-Dw1, an ultra-diffuse galaxy — made of relatively few, widely spread stars — located about 115 million light-years from Earth. It is the first stellar stream ever found around a galaxy other than our own, giving researchers a new way to trace dark matter, the invisible substance that makes up roughly 85% of the universe's matter but neither emits nor reflects light. The findings are reported in the journal Nature.

The stream formed when the host galaxy's gravity pulled stars away from a globular cluster — a dense, gravitationally bound group of stars that orbits a galaxy. Escaped stars tend to trail close to the cluster's original orbit rather than scatter randomly, stretching into a narrow stream that can survive for billions of years. Because the stream's shape is set by the galaxy's gravity, it reveals how the galaxy's mass — including mass no telescope can see directly — is distributed.

The research — co-led by Julie Kiel Holm of the University of Copenhagen and Sarah Pearson of the Technical University of Denmark, and including Northwestern astrophysicist Tjitske Starkenburg — spotted the faint arc in archival Hubble Space Telescope images. UGC 9050-Dw1 was already notable for its distorted shape and roughly 52 globular clusters, which made the new stream easier to detect against its dim background.

"The stars in a stellar stream all travel along nearly the same orbit, and that orbit is shaped by the galaxy's gravity," Starkenburg said. "By modeling that gravity, we can estimate the galaxy's total mass. We already know roughly how much of that mass comes from visible matter like stars, so the rest must be dark matter." The team ran thousands of computer simulations to match the stream's shape, producing what they describe as the first constraint on an ultra-diffuse galaxy's dark matter halo derived from a stellar stream.