Astronomers have discovered the first confirmed stellar stream outside the Milky Way—a faint, narrow ribbon of stars left behind as an ancient globular cluster slowly dissolved over billions of years. The delicate structure offers a powerful new way to study dark matter and the gravitational forces shaping distant galaxies.
An international team of researchers, including astrophysicists from Northwestern University, has identified the first globular cluster stellar stream observed beyond the Milky Way. Astronomers have long predicted that similar stellar streams should exist around other galaxies, but their extreme faintness has made them difficult to detect.
The newly discovered stellar stream also provides an important new tool for investigating dark matter, one of the biggest unanswered questions in astrophysics. By analyzing the stream’s shape and movement, researchers reconstructed the gravitational field of its host galaxy and determined how invisible dark matter influenced the stars’ orbits.
The discovery could eventually help scientists map the distribution of dark matter in many different galaxies and improve their understanding of the universe’s large-scale structure.
The study was published Aug. 12 in the journal Nature.
“The stars in the stream are all moving along roughly the same orbits, and their orbits are shaped by the galaxy’s gravity,” said study co-author Tsitske Starkenburg of Northwestern University. “By modeling its gravity, we can estimate the total mass of the galaxy. We already know how much of that mass comes from visible matter, such as stars, so the remainder must be dark matter.”
Starkenburg is an expert in extragalactic astronomy and a research assistant professor at Northwestern University’s Interdisciplinary Exploration and Research Center in Astrophysics. The study was co-led by Julie Kiel-Holm of the University of Copenhagen and Sarah Pearson of the Technical University of Denmark.
First stellar stream discovered beyond the Milky Way
A globular cluster is a dense group of stars held together by gravity. As a globular cluster travels around its host galaxy, the galaxy’s gravitational forces can gradually pull stars away from the cluster.
These stars do not disperse randomly. Instead, they continue traveling along similar orbits, forming long, narrow stellar streams that preserve information about the gravitational environment they passed through.
Astronomers have identified dozens of stellar streams created by globular clusters within the Milky Way. Until now, however, no comparable stream had been conclusively detected in another galaxy. These structures are typically so faint that they are hidden by the much brighter light of their host galaxies.
The groundbreaking discovery was made using archival observations from NASA’s Hubble Space Telescope. Study co-authors David Sand and Katherine Fielder, astronomers at the University of Arizona, announced the finding. While examining images of the ultra-diffuse galaxy UGC 9050-Dw1, study co-author David Handel noticed a faint, narrow arc that appeared to be a stellar stream.
UGC 9050-Dw1 is located approximately 115 million light-years from Earth. Because the galaxy contains relatively few stars, its darker background made the extremely faint stream easier to distinguish.
A new way to measure dark matter
The significance of the discovery extends beyond identifying the stellar stream itself. For the first time, researchers have demonstrated that globular cluster streams can be used to study dark matter in galaxies outside the Milky Way.
Dark matter accounts for approximately 85% of all matter in the universe. It cannot be observed directly because it does not emit, absorb, or reflect light. Astronomers instead detect dark matter through its gravitational effects on stars, galaxies, and other visible objects.
After identifying the stream, the research team ran thousands of computer simulations. They tested different dark matter distributions and globular cluster properties to determine which scenarios best reproduced the stream’s observed structure and motion.
The model that most closely matched the observations provided a new estimate of UGC 9050-Dw1’s total mass and revealed how that mass is distributed throughout the galaxy. The results indicate that UGC 9050-Dw1 contains a substantial amount of dark matter, as expected for an ultra-diffuse galaxy.
“Our results are consistent with previous studies and with what has been shown about dark matter in this ultra-diffuse galaxy,” Kiel-Holm said. “We are measuring it with a completely new tool for this type of galaxy and demonstrating that the method works beyond the Milky Way.”
Stellar streams may reveal the structure of dark matter
Although the current research focuses on a single galaxy, the discovery could lead to searches for similar stellar streams around many different types of galaxies. With larger samples, astronomers may learn more about how dark matter behaves and how it is distributed across the universe.
Thin stellar streams are especially valuable because even small concentrations of dark matter can disturb them, creating visible gaps, clumps, or other irregularities.
“Narrow stellar streams can develop gaps and clumps when low-concentration structures of dark matter pass through them,” Starkenburg said. “Astronomers have long debated whether we have observed this process in the stellar streams of the Milky Way. If we can confirm that dark matter is responsible for these features, we will have a completely new way to test how dark matter is distributed and ultimately learn more about its fundamental properties.”
New telescopes could discover more stellar streams
Future observatories could make it significantly easier to find these elusive structures. The European Space Agency’s Euclid mission and NASA’s Nancy Grace Roman Space Telescope are designed to survey much larger areas of the sky than Hubble, increasing the likelihood of detecting stellar streams around distant galaxies.
“It is exciting that we were able to use existing Hubble Space Telescope data to discover a thin stream of stars around a galaxy beyond the Milky Way and confirm it with ground-based telescope observations,” Starkenburg said. “We are especially excited about the possibilities offered by new observatories, including the Roman Space Telescope, which will be able to survey roughly 100 times more area than Hubble.”
The study, titled “Evidence for the First Globular Cluster Stellar Stream Beyond the Milky Way,” was supported by VILLUM FONDEN through award VIL53081 and the European Union through the BeyondSTREAMS award 101115754. Starkenburg also acknowledged support from the National Science Foundation through grant AST-2510183 and NASA through grants 22-ROMAN22-0055 and 22-ROMAN22-0013.
Source: www.sciencedaily.com