This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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. 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 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.
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