Ar-Rehman

First globular cluster stellar stream ever identified

Most of the stars in our Milky Way galaxy sit neatly on a flat plane. But the space around our galaxy is much more chaotic. Rogue bands of stars called "stellar streams" orbit the Milky Way much like planets in our solar system orbit the sun. A ghostly stream of stars beyond the Milky Way is giving astronomers a new way to map the universe’s invisible dark matter. A faint ribbon of stars around a distant galaxy has become the first globular cluster stellar stream ever identified beyond the Milky Way. Because the stars trace the galaxy’s gravity, researchers used the stream to estimate how much invisible dark matter surrounds it and how that matter is distributed. The breakthrough offers astronomers a promising new way to investigate one of the universe’s greatest mysteries. Astronomers have long been fascinated by the possibility that stellar streams could indirectly reveal the presence of dark matter, that mysterious theorized substance which doesn't interact with light or normal matter, except via gravity. However, a new University of Washington study casts doubt on a leading theory linking dark matter and stellar streams and raises new questions about both galactic phenomena. "Dark matter makes up most of the mass in the universe and forms the scaffolding that galaxies grow on, but we still don't know what it is," said co-author Nora Shipp, a UW assistant professor of astronomy. "The Milky Way is one of the best laboratories we have for figuring that out, and stellar streams are one of the sharpest tools inside it."

For billions of years, an ancient cluster of stars has been gradually coming apart, shedding stars that now form a faint, narrow ribbon across space. That delicate structure is offering astronomers a new way to investigate one of the universe's biggest mysteries. An international research team, including an astrophysicist from Northwestern University, has identified the first stellar stream of this type ever observed outside the Milky Way. Astronomers have long predicted that such streams should exist around other galaxies, but their extreme faintness has made them difficult to detect. The newly discovered stream also offers scientists an unusual tool for studying dark matter, which remains one of the major unanswered questions in astrophysics. By analyzing the stream's shape, researchers reconstructed the gravitational field of its host galaxy and used that information to determine how unseen dark matter influenced the stars' paths. A stellar stream forms when a group of stars crashes into a galaxy and becomes ensnared by its gravity. As the stars orbit the galaxy, its gravity stretches the cluster into a long, thin filament of stars. Most galaxies host stellar streams, though the Milky Way's are the most visible to astronomers. In our galaxy, most stellar streams we can see are irregular, gaps and kinks interrupt an otherwise uniform smear of stars. Many astronomers believe those irregularities could signal the gravitational tug from small clumps of dark matter, called subhalos. If there are indeed subhalos sprinkled throughout the galaxy, studying the aberrations in stellar streams could teach us about the composition of dark matter.

The new study was an effort to understand the role that the host galaxy, rather than the dark matter clumps within it, plays in shaping stellar streams. Astronomers simulated four Milky Way-sized galaxies without any dark matter clumps, then peppered them with roughly 15,000 stellar streams. After five billion simulated years, the team observed irregularities in nearly every stellar stream. "In our simulations, the host galaxies alone caused the same kinds of irregularities that we observe in real stellar streams," said lead author Arpit Arora, a UW postdoctoral scholar in astronomy. "Now that we can predict what the host galaxy does on its own, we can start isolating the effects for which dark matter is responsible." The findings could eventually help scientists investigate how dark matter is distributed across many different galaxies and throughout the universe. "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's Tjitske Starkenburg. "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." Starkenburg, an expert in extragalactic astronomy, is a research assistant professor at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics.

The cause of the irregularities was the structure of the galaxies themselves. In each simulated galaxy, stars were spread somewhat unevenly across the disk, creating areas of greater and lesser density to mimic the composition of a real galaxy like ours. As the simulated streams of stars passed through denser regions of space, they were bent and torn by the irregular gravitational landscape. Arora expected the host galaxies to impart some irregularities on the streams, but the sheer number caught him off guard. "We found that almost all of the streams had some sort of structural variation," Arora said. "So this idea that streams are naturally thin and smooth wasn't really necessarily true." The simulation generated wiggles, kinks, spurs, branches, gaps and clumps; some streams were totally torn apart by the gravitational froth of their host galaxies. Streams orbiting closer to the galactic core were thrown into dense, clumpy regions of space more often, where they acquired more irregularities. Out of the 15,000 streams spread across the four host galaxies, only 70 remained perfectly smooth after five billion years. The results might seem disheartening, but the UW team believes they chart a clear and exciting course for the future of dark matter research. Arora wants to include dark matter clumps in the next simulation to see whether they produce stellar stream irregularities that are distinct from those caused by the host galaxy alone.

Globular clusters are densely packed groups of stars held together by gravity. As one of these clusters travels around its host galaxy, the galaxy's gravitational pull can slowly strip stars away from it. Those stars do not simply scatter in every direction. Instead, they tend to continue along nearly the same orbital path, creating long, thin stellar streams that retain information about the gravitational forces they have experienced. Astronomers have identified dozens of streams produced by globular clusters inside the Milky Way. Until now, however, no comparable stream had been detected in another galaxy. Such structures are generally so faint that they disappear against the light of their host galaxies. There may also be opportunities to check simulations against new observations: the Simonyi Survey Telescope at the NSF-DOE Vera C. Rubin Observatory is expected to find many more stellar streams within our galaxy, which will help astronomers build a taxonomy of stream features and, hopefully, discover fingerprints of dark matter. "Sadly there's no magic wand to reveal the structure of dark matter," said James Davenport, a research assistant professor of astronomy at the UW. "Streams are complex systems, but they're still the most interesting way to study dark matter close to home." The breakthrough came through archival observations from NASA's Hubble Space Telescope presented by study coauthors David Sand and Catherine Fielder, both astronomers at the University of Arizona. While study coauthor David Hendel was examining images of the ultra-diffuse galaxy UGC 9050-Dw1 for their publication, he noticed a faint, narrow arc that looked like a stellar stream. UGC 9050-Dw1 lies roughly 115 million light-years from Earth. Because the galaxy contains relatively few stars, it provided an unusually dark background that made the dim stream easier to distinguish.

Although the current analysis centers on just one galaxy, the finding could lead to searches for similar stellar streams around many different types of galaxies. With a larger sample, astronomers may be able to learn much more about how dark matter behaves and how it is distributed. The importance of the discovery extends beyond finding the stream itself. Researchers have now shown for the first time that a globular cluster stellar stream can be used to investigate dark matter in a galaxy beyond the Milky Way. Dark matter accounts for roughly 85% of all matter in the universe. Because it does not emit or reflect light, astronomers cannot see it directly. Instead, they detect its effects through the gravitational influence it exerts on stars, galaxies, and other visible objects. Once the stream was identified, the team ran thousands of computer simulations. The researchers tested different combinations of globular cluster characteristics and possible dark matter distributions to determine which scenarios could reproduce the stream's observed appearance. The models that most closely matched the observations provided new estimates of UGC 9050-Dw1's total mass and showed how that mass is distributed throughout the galaxy. The results indicated that UGC 9050-Dw1 contains a large amount of dark matter, as astronomers had expected for an ultra-diffuse galaxy.

Future observatories could make these elusive structures much easier to find. The European Space Agency's Euclid mission and NASA's Nancy Grace Roman Space Telescope are designed to survey far larger regions of the sky than Hubble, increasing the likelihood that astronomers will detect stellar streams around additional galaxies. "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," Starkenburg said. "That makes it very promising for the new telescopes becoming available, including the Roman Space Telescope, which can see an area 100-times larger than that of the Hubble." Co-authors from the UW astronomy department include Peter Ferguson, a postdoctoral fellow; Videep Reddy, an undergraduate student; and Jack Kohm and Laurella Marin, graduate students. This research was funded by the Gordon and Betty Moore Foundation.