For billions of years, an ancient star cluster has been slowly unraveling, leaving behind a delicate ribbon of stars. And now it's finally giving up its secrets.
Discovered by an international team of scientists, including a Northwestern University astrophysicist, this faint trail of stars is the first stellar stream of its kind ever identified outside the Milky Way. Astronomers have long expected such streams to exist in other galaxies, but, until now, had never observed one.
The discovery also provides a powerful way to study dark matter — one of the greatest unsolved mysteries in modern astrophysics. By modeling the stream's shape, the research team reconstructed the galaxy's gravitational field and, in turn, inferred how dark matter shaped the stars' orbits.
The work paves the way for broader investigations into how this elusive, invisible matter is distributed throughout the universe.
The study will be published on Wednesday (Aug. 12) in the journal Nature.
"The stars in a stellar stream all travel along nearly the same orbit, and that orbit is shaped by the galaxy's gravity," said Northwestern's Tjitske Starkenburg , who coauthored the study. "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."
An expert in extragalactic astronomy, Starkenburg is a research assistant professor at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics . The study was co-led by Julie Kiel Holm of the University of Copenhagen and Sarah Pearson of the Technical University of Denmark.
A first beyond our galaxy
Bound together by gravity, globular clusters are dense collections of stars. As they orbit their host galaxy, the galaxy's gravity gradually pulls stars away from the cluster. Rather than dispersing randomly, those escaped stars continue traveling along nearly the same orbit — forming long, narrow stellar streams that preserve a record of the gravitational forces that shaped them.
Although astronomers have discovered dozens of globular cluster stellar streams within the Milky Way, they previously were unable to identify such streams other galaxies. Typically, extragalactic streams from globular clusters are simply too faint to see.
But then study coauthors David Sand and Catherine Fielder — who are both astronomers at the University of Arizona — presented archival data from NASA's Hubble Space Telescope. As study coauthor David Hendel examined these images of an ultra-diffuse galaxy called UGC 9050-Dw1 in their publication, he spotted a faint, thin arc that resembled a stellar stream. Located roughly 115 million light-years from Earth, the galaxy's sparse population of stars created a relatively dark backdrop, making the faint stream easier to detect.
A sensitive tool for mapping dark matter
The discovery's significance lies not only in the detection. For the first time, scientists also demonstrated they can use globular cluster stellar streams as a tool for measuring dark matter in other galaxies. Dark matter makes up roughly 85% of all matter in the universe. But because it neither emits nor reflects light, scientists cannot observe dark matter directly. Instead, they must use tools to infer its presence.
After identifying the stream, the scientists generated thousands of computer simulations to determine what combination of globular cluster properties and dark matter distributions could reproduce its observed shape. The best-fitting models revealed new estimates of the galaxy's total mass and how that mass is distributed. Ultimately, the analyses indicated UGC 9050-Dw1 contains large amounts of dark matter, which the astronomers expected for an ultra-diffuse galaxy.
"Our results are consistent with previous studies and what they have shown about dark matter in this ultra-diffuse galaxy," Kiel Holm said. "We are measuring it with a completely new tool for this type of galaxy, demonstrating that this method also works beyond our own galaxy."
Toward the next generation of telescopes
Although the study focuses on a single galaxy, the discovery opens the door to detecting stellar streams across a wide range of galaxy types. By studying more samples, astrophysicists hope to get closer to understanding the mysterious nature and behavior of dark matter.
"Thin stellar streams can develop gaps or clumps when small concentrations of dark matter pass through them," Starkenburg said. "Astronomers have long debated whether we've seen this happen in streams within the Milky Way. If we can confirm that's what's causing these features, that will give us an entirely new way to test how dark matter is distributed — and ultimately learn more about its fundamental nature."
Upcoming observatories, including the European Space Agency's Euclid mission and NASA's Nancy Grace Roman Space Telescope, will survey much larger areas of the sky than Hubble. These new technologies will dramatically increase the chances of finding similar streams in other galaxies.
"It's exciting that we discovered a thin stellar stream around a galaxy other than our own with already-existing Hubble Space Telescope data and confirmed it with ground-based telescope data," Starkenburg said. "That makes it very promising for the new telescopes becoming available, including the Roman Space Telescope, which can see an area 100-times larger than that of the Hubble."
The study, "Evidence for the First Globular Cluster Stellar Stream beyond the Milky Way," was supported by VILLUM FONDEN (award number VIL53081) and the European Union (BeyondSTREAMS award number 101115754). Starkenburg also gratefully acknowledges support from the NSF (grant number AST-2510183) and NASA (grant numbers 22-ROMAN22-0055 and 22-ROMAN22-0013).
