Astronomers have identified the first stellar stream from a globular cluster outside the Milky Way. The faint ribbon of stars appears in deep Hubble Space Telescope images of the ultra-diffuse galaxy UGC 9050-Dw1, about 115 million light-years from Earth, and its shape is already being used to estimate how much dark matter surrounds the galaxy.
Globular clusters are dense groups of stars held together by gravity. As one of these clusters circles a galaxy, the galaxy’s tidal pull can strip stars away. The escaped stars do not scatter at random. They continue along nearly the same orbit, leaving a long, narrow stream that preserves the gravitational forces acting on the cluster.
Those streams are common in the Milky Way, where astronomers have found many examples. Beyond our galaxy they are much harder to see because they are extremely faint. In this case, the sparse background of UGC 9050-Dw1 made the stream easier to spot in archival Hubble observations. A thin arc near a compact star cluster stood out as a possible trail of stars being pulled apart.
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UGC 9050-Dw1 belongs to a class known as ultra-diffuse galaxies. They can be as wide as much brighter galaxies but contain far fewer stars, which makes their mass difficult to measure with ordinary tools such as rotation curves or stellar velocities. The stream provides a different route: its curvature and brightness record the orbit of the cluster that created it and the gravitational field through which that orbit has travelled.
The team compared the observed stream with thousands of computer simulations. Each simulation tested different combinations of cluster properties and dark-matter distributions. The models that best matched the stream’s shape pointed to a massive dark-matter halo around the galaxy, consistent with earlier estimates but measured here with a new method.
The stream’s appearance also supports the idea that it came from a globular cluster rather than from a loose spray of ordinary stars. Its morphology, colour and apparent connection to a compact source fit the cluster interpretation. The study’s modelling reproduced the observed surface brightness and used the stream to place the first stream-based constraint on the halo of an ultra-diffuse galaxy.
Dark matter cannot be seen directly because it does not emit or reflect light. Astronomers infer its presence from gravity: from the way stars move, galaxies rotate and light bends as it travels through space. A stellar stream adds another kind of record. Its orbit carries information about the invisible mass that has been shaping the galaxy for billions of years.
The result is still based on one external galaxy, so it is not a complete survey of how dark matter is distributed. But it shows that globular-cluster streams can be used outside the Milky Way, where other measurements are often difficult because ultra-diffuse galaxies contain relatively few bright stars. Future observations could reveal more streams and test whether small gaps or clumps in them are caused by concentrations of dark matter.
Those gaps would be especially valuable. A small concentration of dark matter passing through a stream can pull stars away from the ribbon and leave a measurable interruption. If astronomers find the same pattern in several galaxies, they will have a way to compare dark-matter substructure far beyond the neighbourhood of the Milky Way.
For Curiosmos readers, the discovery connects two scales that are usually discussed separately: the slow destruction of a star cluster and the structure of an entire galaxy. It also complements our coverage of the Milky Way’s early mergers, how astronomers measure cosmic distances and the evidence scientists use to study invisible parts of the universe.
Sources and further reading: Evidence for the First Globular Cluster Stellar Stream beyond the Milky Way (research paper); Northwestern University research report.
