NASA’s star stream data reveals how dark matter shapes galaxy orbits

The first stellar stream from a globular cluster outside the Milky Way has been identified, a discovery offering a new way to map dark matter in distant galaxies. An international team, including Northwestern University astrophysicist Tjitske Starkenburg, detected the faint ribbon of stars in the ultra-diffuse galaxy UGC 9050-Dw1, located 115 million light-years from Earth.

“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 this gravity, researchers estimate the galaxy’s total mass and, crucially, the amount of unseen dark matter present.

Globular Cluster Stellar Stream Discovered Beyond the Milky Way

The first detection of a globular cluster stellar stream outside the Milky Way confirms a long-held theoretical prediction and unlocks a new method for mapping dark matter distribution in distant galaxies. Located 115 million light-years away, the faint stream orbits UGC 9050-Dw1, an ultra-diffuse galaxy where a sparse stellar population aided in its discovery using archival data from the Hubble Space Telescope.

This finding, detailed in Nature, moves beyond observations limited to our own galaxy and establishes a technique for probing the invisible mass that dominates the universe. Researchers, led by Julie Kiel Holm of the University of Copenhagen and Sarah Pearson of the Technical University of Denmark, utilized computer simulations to model the stream’s shape and reconstruct the galaxy’s gravitational field.

By analyzing these orbits, the team estimated UGC 9050-Dw1’s total mass, revealing a substantial presence of dark matter consistent with previous studies of similar galaxies. Kiel Holm added, “Our results are consistent with previous studies and what they have shown about dark matter in this ultra-diffuse galaxy.”

The significance of this discovery extends beyond simply confirming the existence of extragalactic stellar streams; it demonstrates the viability of using these streams as tools to measure dark matter. Dark matter, comprising approximately 85% of the universe’s total matter, remains elusive due to its lack of interaction with light, necessitating indirect methods of detection. The team believes that subtle disturbances within these streams, such as gaps or clumps, could reveal even more about the nature of dark matter itself.

“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.” The current findings are particularly promising given the capabilities of forthcoming telescopes, like the Nancy Grace Roman Space Telescope, which will survey areas 100 times larger than Hubble. Starkenburg noted, “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,” which makes it promising for the new telescopes becoming available.

Hubble Data Models Dark Matter’s Influence on Galactic Orbits

Starkenburg explained how the team leveraged the stream’s orbital path to model the host galaxy’s gravity. By generating thousands of computer simulations, researchers determined the combination of globular cluster properties and dark matter distributions that best matched the observed stream’s shape, ultimately estimating UGC 9050-Dw1’s total mass. The analyses indicated the galaxy contains substantial amounts of dark matter, consistent with expectations for ultra-diffuse galaxies. We already know roughly how much of that mass comes from visible matter like stars, so the rest must be dark matter,” Starkenburg added.

This technique represents a significant advancement because it allows scientists to infer dark matter’s presence without directly observing it. The team demonstrated the method’s applicability beyond the Milky Way, a crucial step toward understanding dark matter’s distribution across diverse galactic environments.