Astronomers Simulated 15,000 Star Streams With No Dark Matter Clumps at All. Only 70 Came Out Smooth.

One of the most promising ways to find dark matter close to home just got much harder to interpret. A University of Washington study published Aug. 27 in The Astrophysical Journal simulated four Milky Way-sized galaxies containing no dark matter clumps whatsoever, seeded them with roughly 15,000 stellar streams, and ran them forward five billion years. Nearly every stream came out scarred. Out of 15,000, only 70 remained perfectly smooth.

Stellar streams are the wreckage of galactic cannibalism. When a star cluster or dwarf galaxy falls into a larger galaxy, the host's gravity stretches it into a long, thin filament of stars that keeps orbiting like a smeared-out ribbon. The Milky Way's streams are the best-resolved in the universe simply because we sit inside the galaxy. And for years, the gaps, kinks and spurs that interrupt those ribbons have been treated as one of the sharpest available probes of dark matter — the reasoning being that a passing clump of dark matter, called a subhalo, would tug on the stream and leave a dent.

"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 new work asked a control question that had gone under-tested: what does the host galaxy alone do to a stream? In the simulations, stars were spread unevenly across each galactic disk, creating denser and thinner patches, the way a real galaxy's spiral arms and clumps do. As streams passed through the dense regions, the uneven gravitational landscape bent and shredded them. The simulations produced wiggles, kinks, spurs, branches, gaps and clumps — the full catalog of features attributed to dark matter — and in some cases tore streams apart entirely. Streams orbiting nearer the galactic core, where the disk is densest, were hit hardest.

"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. He had expected the host to leave some mark. He had not expected it to leave a mark on essentially everything. "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."

That does not close the door on stream-based dark matter hunting; it reframes it as a subtraction problem. The team's argument is that you cannot attribute a gap to a dark matter subhalo until you know what the galaxy would have done to that stream without one. "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," Arora said. His next step is to rerun the simulations with subhalos included, to see whether dark-matter-induced damage carries a signature distinguishable from the host's.

The observational side is about to get much richer. The Simonyi Survey Telescope at the NSF-DOE Vera C. Rubin Observatory is expected to turn up many more Milky Way streams, enough to build a working taxonomy of stream features and test which categories, if any, the host galaxy cannot produce on its own. "Sadly there's no magic wand to reveal the structure of dark matter," said James Davenport, a UW research assistant professor of astronomy. "Streams are complex systems, but they're still the most interesting way to study dark matter close to home."