Astronomers at the University of Copenhagen have made a landmark discovery: the first globular cluster stellar stream ever observed in a galaxy beyond the Milky Way. The finding, published in Nature, demonstrates that these faint ribbons of stars can be used as a powerful new tool to map dark matter in distant galaxies.
The stream was identified in UGC9050-Dw1, an ultra-diffuse galaxy that emits very little light. Globular cluster stellar streams are coherent structures of stars that retain a record of their dynamical history, offering unique insights into galaxy evolution and dark matter behavior.
"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, the Milky Way," said lead author Julie Kiel Holm, a PhD student at the Niels Bohr Institute.
The discovery is particularly remarkable because these streams are extremely difficult to detect beyond the Milky Way due to their faint signal. The team combined advances in large astronomical datasets with sophisticated analysis techniques to make the detection.
The researchers demonstrated that globular cluster stellar streams can constrain the dark matter halo mass and density slope of external galaxies—measurements that have traditionally been very challenging. Their analyses indicate the galaxy contains large amounts of dark matter, consistent with expectations for ultra-diffuse galaxies.
"We show that a well-established tool from studies of the Milky Way can be used to understand other galaxies, where measuring the distribution of dark matter has traditionally been very challenging," said Holm.
Dark matter makes up roughly 25% of the universe's mass-energy content, yet its nature remains unknown. The Standard Model of particle physics cannot explain it. By establishing stellar streams as a practical tool beyond the Milky Way, researchers may eventually map dark matter across many galaxy types.
"The insights into dark matter that we have previously been able to gain from globular cluster stellar streams have been limited to a single galaxy—our own," said co-author Sarah Pearson from DTU Space. "Being able to observe these streams in entirely different kinds of galaxies opens the door to building a much broader understanding of how dark matter behaves."
With upcoming facilities like the Euclid Space Telescope and Nancy Grace Roman Space Telescope, the number of observable stellar streams is expected to increase dramatically, potentially transforming dark matter research.
