Astronomers find first stellar stream beyond the Milky Way

An international team of astronomers has discovered the first globular cluster stellar stream ever observed outside the Milky Way, opening a new window into one of the universe's deepest mysteries: dark matter.

A Faint Trail of Stars in a Distant Galaxy

The stream, a delicate ribbon of stars being stripped from an ancient star cluster, was identified in an ultra-diffuse galaxy called UGC 9050-Dw1, located roughly 115 million light-years from Earth. The findings, published Wednesday in Nature, represent a milestone in extragalactic astronomy.eurekalert+1

"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, which makes the discovery particularly exciting," said Julie Kiel Holm, a PhD student at the University of Copenhagen's Niels Bohr Institute who co-led the study with Sarah Pearson of the Technical University of Denmark.eurekalert

The discovery was hiding in plain sight — in archival data from NASA's Hubble Space Telescope. Study co-authors David Sand and Catherine Fielder of the University of Arizona had collected the images, and colleague David Hendel spotted the faint, thin arc resembling a stellar stream.news.northwestern

A New Tool for Measuring Dark Matter

The discovery's broader importance lies in its demonstration that globular cluster stellar streams can serve as tools for measuring dark matter in galaxies beyond our own. Dark matter constitutes roughly 85% of all matter in the universe but neither emits nor reflects light, making it invisible to direct observation.news.northwestern+1

By generating thousands of computer simulations, the team determined what combination of cluster properties and dark matter distributions could reproduce the stream's observed shape. Their best-fitting models revealed that UGC 9050-Dw1 contains large amounts of dark matter, consistent with expectations for ultra-diffuse galaxies.news.northwestern

"The stars in a stellar stream all travel along nearly the same orbit, and that orbit is shaped by the galaxy's gravity," said Northwestern University's Tjitske Starkenburg, a co-author. "By modeling that gravity, we can estimate the galaxy's total mass. We already know roughly how much of that mass comes from visible matter like stars, so the rest must be dark matter."news.northwestern

Looking Ahead

The researchers expect upcoming observatories — including the European Space Agency's Euclid mission and NASA's Nancy Grace Roman Space Telescope — to dramatically increase the number of detectable stellar streams in other galaxies. Starkenburg noted that the Roman Space Telescope can observe an area 100 times larger than Hubble's field of view.news.northwestern

"Thin stellar streams can develop gaps or clumps when small concentrations of dark matter pass through them," Starkenburg said. "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."news.northwestern