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A new study suggests that the twisted, kinked trails of stars circling our galaxy may not be evidence of invisible dark matter.
Rethinking the Origins of Galactic Kinks
Published Aug. 27 in The Astrophysical Journal, new research from the University of Washington reveals that stellar stream irregularities traditionally attributed to dark matter can actually be caused by the host galaxy itself. Researchers found that simulated galaxies without dark matter clumps still produced twists and kinks in orbiting streams of stars.
Most dark matter simulations focus on large-scale cosmic structure and how dark matter affects the clustering of galaxies over time. In contrast, researchers simulated stellar streams around virtual host galaxies to test theories about galactic shape.
Building Virtual Milky Ways to Isolate Host Gravity
Stellar streams form when dwarf galaxies or star clusters collide with larger galaxies, leaving long, thin filaments of stars orbiting outside the galactic plane. The Milky Way hosts at least two dozen such streams, while astronomers have found several around the Andromeda galaxy.
To isolate the role of the host galaxy, lead author Arpit Arora and colleagues simulated four Milky Way-sized galaxies treating dark matter as a simple, uniform halo rather than varying clumps. The team then peppered these virtual galaxies with roughly 15,000 stellar streams. After five billion simulated years, nearly every stream developed irregularities.
How Regular Matter Bends and Tears Star Filaments
Regular matter alone is enough to cause kinks and twists within a stellar stream. Researchers previously thought these deformations stemmed from small clumps of dark matter called subhalos within the galactic halo.
Instead, the structure of the host galaxies caused the deformations. Stars in the simulated galaxies were spread unevenly across the disc to mimic real galactic composition. As stellar streams passed through denser regions of space, the irregular gravitational landscape bent and tore them.
The kinking effect proved strongest for streams orbiting closer to the galactic center, but even distant streams developed deformations, making smooth streams rare. The simulated irregularities closely resemble real streams observed around the Milky Way.
Shifting Focus Toward the Vera Rubin Telescope
Because the simulations generated twists, kinks, and clumps similar to real observations, large features within Milky Way streams likely cannot be used to study dark matter clumping directly right now.
However, upcoming observations from the Vera Rubin telescope will capture extensive data on faint streams residing on the outer edges of the Milky Way. If those distant streams exhibit strong deformation effects, that data could point toward actual dark matter interactions.
“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,” said co-author Nora Shipp, a University of Washington assistant professor of astronomy, in a release from the institution. The study provides astronomers with new baselines to compare against reality as observational data expands.
Pro Tip: Tracking Galactic Evolution
When analyzing stellar streams, researchers must account for the gravitational impact of the host galaxy’s disk density before attributing structural gaps to dark matter subhalos.
Frequently Asked Questions
What are stellar streams?
Stellar streams are long, thin filaments of stars that form when a dwarf galaxy or star cluster collides with a larger galaxy and gets stretched out by gravitational interactions.
Do stellar streams prove the existence of dark matter?
While stellar streams interact with dark matter via gravity, a University of Washington study published in The Astrophysical Journal shows that regular matter within a host galaxy can cause the same kinks and twists previously thought to be created by dark matter clumps.
How do researchers plan to isolate dark matter effects?
By establishing baseline simulations of how regular host galaxies shape stellar streams on their own, astronomers can better filter out those background effects and identify true dark matter signals using data from instruments like the Vera Rubin telescope.
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