Stars Travel More Slowly At Milky Way’s Edge

Wednesday, January 31, 2024
by Jennifer Chu

By clocking the speed of stars throughout the Milky Way galaxy, MIT physicists have found that stars further out in the galactic disk are traveling more slowly than expected compared to stars that are closer to the galaxy’s center. The findings raise a surprising possibility: The Milky Way’s gravitational core may be lighter in mass, and contain less dark matter, than previously thought.

The new results are based on the team’s analysis of data taken by the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise location, distance, and motion of more than 1 billion stars throughout the Milky Way galaxy, while APOGEE is a ground-based survey. The physicists analyzed Gaia’s measurements of more than 33,000 stars, including some of the farthest stars in the galaxy, and determined each star’s “circular velocity,” or how fast a star is circling in the galactic disk, given the star’s distance from the galaxy’s center.

“What we were really surprised to see was that this curve remained flat, flat, flat out to a certain distance, and then it started tanking,” says Lina Necib, assistant professor of physics at MIT. “This means the outer stars are rotating a little slower than expected, which is a very surprising result.”

The team translated the new rotation curve into a distribution of dark matter that could explain the outer stars’ slow-down, and found the resulting map produced a lighter galactic core than expected. That is, the center of the Milky Way may be less dense, with less dark matter, than scientists have thought.

“This puts this result in tension with other measurements,” Necib says. “There is something fishy going on somewhere, and it’s really exciting to figure out where that is, to really have a coherent picture of the Milky Way.”

The team reports its results this month in the Monthly Notices of the Royal Society Journal. The study’s MIT co-authors, including Necib, are first author Xiaowei Ou, Anna-Christina Eilers, and Anna Frebel.

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