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.
Astronomers have long been fascinated by the possibility that stellar streams could indirectly reveal the presence of dark matter, that mysterious theorized substance that 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.”
The study was published Aug. 27 in The Astrophysical Journal.
Take a stellar stream tour
Use the visualizer below to explore some of the simulated stellar streams from the study. Click and drag the image to rotate the view. Scroll to zoom. Click or tap the gear icon to access variables like number of streams, rotation rate and more. Use the icon in the lower lefthand corner to go fullscreen.
Credit: Visualization by Arpit Arora and Adrian Price-Whelan. Milky Way image credit: Stefan Payne-Wardenaar.
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 that 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 part that dark matter is responsible for.”
The cause of the irregularities was the structure of the galaxies themselves. In each simulated galaxy, stars were spread somewhat unevenly across the disc, 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.
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 the dark matter close to home.”
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.
A complete list of co-authors is included with the paper.
This research was funded by the Gordon and Betty Moore Foundation.
For more information, contact Arora at arora125@uw.edu.