Most of the stars in sit neatly on a flat plane. But the space around our galaxy is much more chaotic. Rogue bands of stars called 鈥溾�� 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 , that mysterious theorized substance that doesn鈥檛 interact with light or normal matter 鈥� except via gravity. However, a new 黑料老司机 study casts doubt on a leading theory linking dark matter and stellar streams, and raises new questions about both galactic phenomena.
鈥淒ark 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,鈥� said co-author , a UW assistant professor of astronomy. 鈥淭he 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.鈥�
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 . Most galaxies host stellar streams, though the Milky Way鈥檚 are the most visible to astronomers.
In our galaxy, most stellar streams we can see are irregular 鈥� 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 鈥� rather than the dark matter clumps within it 鈥� 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.
鈥淚n our simulations, the host galaxies alone caused the same kinds of irregularities that we observe in real stellar streams,鈥� said lead author , a UW postdoctoral scholar in astronomy. 鈥淣ow that we can predict what the host galaxy does on its own, we can start isolating the part that dark matter is responsible for.鈥�
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.
鈥淲e found that almost all of the streams had some sort of structural variation,鈥� Arora said. 鈥淪o this idea that streams are naturally thin and smooth wasn’t really necessarily true.鈥�
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 is , which will help astronomers build a taxonomy of stream features and 鈥� hopefully 鈥� discover fingerprints of dark matter.
鈥淪adly there’s no magic wand to reveal the structure of dark matter,鈥� said , a research assistant professor of astronomy at the UW. 鈥淪treams are complex systems, but they’re still the most interesting way to study the dark matter close to home.鈥�
Co-authors from the UW astronomy department include , a postdoctoral fellow; , an undergraduate student; and and , graduate students.
A complete list of co-authors is .
This research was funded by the Gordon and Betty Moore Foundation.
For more information, contact Arora at arora125@uw.edu.