An Evanston-based astrophysicist helped an international team spot something astronomers thought was nearly impossible to detect: a faint ribbon of stars being torn from a star cluster in a galaxy 115 million light-years away.
Tjitske Starkenburg, a research assistant professor at Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), co-authored the study published Wednesday, Aug. 12, in the journal Nature. The finding represents the first globular cluster stellar stream ever identified outside our own galaxy.
The team named the stream Oyashio, after a cold Pacific ocean current off the coast of Japan. It stretches roughly 6,500 light-years long and 236 light-years wide within an ultra-diffuse galaxy called UGC 9050-Dw1.
Hidden in Hubble's archives
The discovery was hiding in plain sight.
Co-author David Hendel first noticed the faint, thin arc while examining archival Hubble Space Telescope images originally collected in September 2022. The team then independently confirmed the stream using three separate exposures from the Canada-France-Hawaii Telescope, ruling out camera artifacts.
The signal stood out at 7.34 standard deviations above the surrounding background in the combined Hubble image.
"The area that the Hubble can see is actually not that big," Starkenburg told CNN. "So you have to be very, very lucky to be able to see this, and we don't really know where to look to basically target them."
A new tool for measuring dark matter
A stellar stream forms when a galaxy's gravity gradually pulls stars away from an orbiting globular cluster. Those escaped stars continue along nearly the same orbit, forming a narrow ribbon that encodes information about the galaxy's gravitational field.
By running thousands of computer simulations to model Oyashio's shape, the team reconstructed UGC 9050-Dw1's gravitational field and estimated its dark matter content. The best-fitting model puts the galaxy's total dark matter halo mass at roughly 156 billion times the mass of our sun, comparable to the Large Magellanic Cloud.
That matters because dark matter makes up an estimated 85% of all matter in the universe but cannot be observed directly. According to the Northwestern press release, Starkenburg said the stars in a stream all travel along nearly the same orbit shaped by the galaxy's gravity, and by modeling that gravity, researchers can estimate total mass and subtract visible matter to infer how much dark matter remains.
The study was co-led by Julie Kiel Holm, a doctoral fellow at the University of Copenhagen's Niels Bohr Institute, and Sarah Pearson, an associate professor at the Technical University of Denmark's National Space Institute. Starkenburg is also affiliated with the NSF-Simons AI Institute for the Sky, and her research was supported by NSF and NASA grants.
What comes next
Researchers say the discovery is especially promising given that NASA's Nancy Grace Roman Space Telescope, scheduled to launch Sunday, Aug. 30, will carry a field of view more than 100 times larger than Hubble's. The Vera C. Rubin Observatory, which began its Legacy Survey of Space and Time on June 30, is also expected to expand the search for similar streams in other galaxies.
