Dark Matter Has Slowed The Spin Of The Milky Way's Galactic Bar

Our Milky Way galaxy has something known as the galactic bar that's made up of billions of clustered stars. Scientists say that the galactic bar has reduced the rate of its spinning by about 25 percent since its formation. New details on the slowed spin rate surfaced in a study published by UCL and University of Oxford researchers.The team says for 30 years, astrophysicists have predicted a slowdown, but the new study marks the first time it has been measured. In addition, the study gives new insight into the nature of dark matter, which acts as a counterweight slowing the spin. Researchers on the team analyzed Gaia space telescope observations of a large group of stars known as the Hercules stream.

Those stars are in resonance with the galactic bar meaning they revolve around the galaxy at the same rate as the bar's spin. Stars are gravitationally trapped by the spinning bar. If the spin of the Galactic bar slows down, the stars would be expected to move further out into the galaxy, keeping their orbital period matched to that of the bar's spin. Stars in the stream have been discovered to carry a chemical fingerprint that shows they are richer in heavier elements proving they have traveled away from the galactic center where stars and star-forming gas are about ten times richer in metals compared to the outer galaxy.

Using that new data, the team could infer that the bar, made up of billions of stars consisting of trillions of solar masses, has slowed its spin by at least 24 percent since it first formed. Researchers say the counterweight slowing spin has to be dark matter. Previously, researchers were only able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible structures.

The research provides a new way to measure dark matter by its inertial mass. Researchers believe the Milky Way is embedded in a "halo" of dark matter extending well beyond the visible edge of the galaxy.