First Globular Cluster Stellar Stream Found Outside the Milky Way

For the first time, astronomers have identified a globular cluster stellar stream in a galaxy beyond the Milky Way — a faint trail of stars being tidally stripped from a globular cluster in the ultra-diffuse galaxy UGC 9050-Dw1, 115 million light-years away. The discovery, published in August 2026 in Nature by a team led by Ph.D. student Julie Kiel Holm (Niels Bohr Institute) and associate professor Sarah Pearson (DTU Space), opens a new observational frontier and provides a powerful new tool for studying dark matter in distant galaxies .

Why This Discovery Matters

While dozens of globular cluster streams are known inside the Milky Way (e.g., GD-1, Palomar 5), none had ever been confirmed in an external galaxy before . This detection proves that the stellar stream technique — long used to map dark matter in our own galactic neighborhood — can work in galaxies millions of light-years away.

Stellar streams are dynamically cold structures: their thin, coherent shape is exquisitely sensitive to gravitational perturbations from dark matter subhalos . By modeling the stream's shape, researchers can infer the geometry and clumpiness of the dark matter halo around the host galaxy, a feat previously impossible beyond the Local Group .

How It Was Found in Archival Hubble Data

The stream was spotted in existing images from the Hubble Space Telescope's Advanced Camera for Surveys (ACS) that were originally taken for other purposes . The research team combed through archival data looking for signs of disturbed morphology around UGC 9050-Dw1 — a galaxy already known to be exceptionally rich in globular clusters (about 52 ± 12 identified) and exhibiting a tidal plume from previous interactions . The team noticed an extended, thin stellar feature stretching away from one of the galaxy's globular clusters, matching the signature of a tidal stream . Northwestern University astrophysicist Catherine Fielder was part of the earlier work characterizing the galaxy's globular cluster population .

UGC 9050-Dw1 is an ultra-diffuse galaxy (UDG), a type of galaxy with very low surface brightness. It hosts an unusually large number of globular clusters — roughly 20% of its total light comes from these dense star clusters .

How Stellar Streams Trace Dark Matter

The morphology of a stellar stream — its width, curvature, and any gaps or wiggles along its length — directly encodes the gravitational field it travels through . If dark matter is "cold" and clumpy, streams will be perturbed by many low-mass subhalos, creating distinctive gaps and kinks. If dark matter is warm or has different properties, the subhalo population changes, producing different stream shapes. By modeling the observed stream in UGC 9050-Dw1, the team has begun testing dark matter models at distances previously impossible .

Using a generative modeling approach called X-Stream, the team directly fitted dynamical models to the stream's shape to constrain the mass of the progenitor globular cluster and presented the first stream-based halo constraint for an ultra-diffuse galaxy . This work suggests UGC 9050-Dw1 possesses a dark matter halo with specific structural properties .

The Coming Revolution: Roman, Euclid, and Rubin

The team expects this detection is just the start. Upcoming missions will dramatically expand the hunt:

• Nancy Grace Roman Space Telescope (NASA) — With its 200-megapixel wide-field imager, Roman can survey entire nearby galaxies in a single shot, resolving thin stellar streams that would require hundreds of Hubble images to cover . Roman is specifically designed to study stellar streams and is predicted to find dozens of globular cluster streams in external galaxies . A NASA Science article notes that Roman "will be an excellent tool for finding thin stellar streams" beyond the Milky Way for the first time .
• Euclid (ESA) — Its wide optical and near-infrared survey will map extragalactic tidal features across large areas of sky, complementing Roman's deeper pointed observations .
• Vera C. Rubin Observatory (LSST) — Simulations show LSST alone could observe 1,000 or more stars in 40–100 globular cluster streams beyond the Milky Way .

European Research Council-funded projects like BeyondSTREAMS and EXTRADARK are already preparing the theoretical frameworks needed to interpret the wealth of incoming stream data from these facilities .

Together, these observatories will transform the handful of known extragalactic streams into a statistical sample, enabling precision dark matter measurements across a wide range of galaxy types and environments .