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 “” 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’t interact with light or normal matter — except via gravity. However, a new ÂŇÂ×ÉçÇř 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 , 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.”

in The Astrophysical Journal.

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’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 , 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 is , 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 , 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 , 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.

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Recent recognition of the ÂŇÂ×ÉçÇř includes the Best Paper Award at NeurIPS Pluralistic Alignment Workshop, Scialog: Early Science with the LSST Collaborative Innovation Award and 2024 AVS Thin Film Young Investigator Award.

Professor wins ‘best paper’ at NeurIPS Pluralistic Alignment Workshop

, assistant professor in the UW Foster School of Business, received the $1,000 Best Paper Award on Pluralistic Alignment at the NeurIPS 2024 Workshop.

The Conference on Neural Information Processing Systems, or NeurIPS, is one of the most influential conferences in artificial intelligence, machine learning and data science and is known for its rigorous peer-review process.

Kleiman-Weiner co-authored the paper, “,” which introduces the MultiTP dataset — a collection of moral dilemmas in over 100 languages that enables the assessment of large language models’ decision-making in diverse linguistic contexts. The analysis explored the alignment of 19 LLMs with human judgments across six moral dimensions.

“By examining moral decisions across over 100 languages, we discovered that language models often fail to capture the rich diversity of human moral preferences across cultures,” Kleiman-Weiner said. “This reinforces why pluralistic alignment — ensuring AI systems can understand and respect different cultural perspectives — is so crucial as we develop these technologies. I’m excited about this work because it pushes us to think critically about whose values AI systems reflect. … We hope this research encourages more work on building AI systems that serve all of humanity, not just a select few.”

Nora Shipp receives Collaborative Innovation Award

, UW assistant professor of astronomy, was part of one of eight interdisciplinary teams awarded the in the first year of Scialog: Early Science with the LSST.

This initiative, launched by the Research Corporation for Science Advancement, is a three-year program designed to support early-career scientists as they prepare to utilize data from the upcoming Legacy Survey of Space and Time, or LSST, at the Vera C. Rubin Observatory in Chile.

“Scialog has been a great opportunity to make connections with scientists across the field of astronomy to brainstorm new ideas for taking advantage of the unprecedented data that will soon be provided by the LSST,” said Shipp.

Shipp’s proposal brings together researchers to study stars and dark matter — not just in the Milky Way, but also in smaller galaxies. By using the LSST to reveal the faint outer regions of these galaxies, the research will help us to better understand the universe’s creation and the limits of how galaxies form.

, which is short for “science + dialog, “is a collaborative program launched by RCSA in 2010. It’s designed to accelerate breakthroughs by fostering a network of creative scientists across disciplines and encouraging intensive discussions on scientific themes of global importance.

As part of this initiative, the conference brought together an expert group of scientists and facilitators, including Eric Bellm, research associate professor of astronomy and DiRAC Institute Fellow, to guide the discussions.

Chemical engineering professor wins 2024 AVS Thin Film Young Investigator Award

, UW assistant professor of chemical engineering, has been named the 2024 recipient of the American Vacuum Society (AVS) . Named in honor of Professor Paul H. Holloway, a distinguished scholar and contributor to AVS, the award recognizes young scientists for significant theoretical and experimental contributions to thin film research.

Bergsman studies how to deposit layers of plastic that are 1/1000 the thickness of a human hair, which he uses to develop better materials for computer processors, clean energy, and water purification.

“The American Vacuum Society was foundational to my growth as a young scientist,” Bergsman said. “I am deeply honored to receive this award from a community which has always been an inspiring and supportive environment. I’m excited to continue engaging with this network of scientists and pushing the boundaries of research in interfacial engineering, surface science, thin films, and related technologies.”

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