Astrophysicists have predicted such a slowdown for 30 years, but this is the first time it has been measured. According to the researchers, it provides new insight into the nature of dark matter, which acts as a counterweight slowing the spin.
According to a new study by researchers at University College London (UCL) and the University of Oxford, the spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation.
According to the researchers, it provides new insight into the nature of dark matter, which acts as a counterweight slowing the spin. The study, published in the Monthly Notices of the Royal Astronomical Society, examined Gaia space telescope observations of a large group of stars known as the Hercules stream that is in resonance with the bar, meaning they revolve around the galaxy at the same rate as the bar’s spin.
For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured. The spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about a quarter since its formation.
The spinning bar has gravitationally trapped these stars. The same thing happens when Jupiter’s Trojan and Greek asteroids orbit Jupiter’s Lagrange points (ahead and behind Jupiter). If the bar’s spin slows, these stars should move further out in the galaxy, keeping their orbital period in sync with the bar’s spin.
The stars in the stream have a chemical fingerprint: they are richer in heavier elements (called metals in astronomy), indicating that they have traveled away from the galactic center, where stars and star-forming gas are about ten times as rich in metals as the outer galaxy.
Using this information, the team calculated that the bar, which is made up of billions of stars and trillions of solar masses, had slowed its spin by at least 24 percent since its formation. “Astrophysicists have long suspected that the spinning bar at the center of our galaxy is slowing down, but we have found the first evidence of this happening,” said co-author Dr. Ralph Schoenrich (UCL Physics & Astronomy).
Dark matter must be the counterweight slowing this spin. We could only infer dark matter until now by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter. Our findings provide a new way to measure dark matter: its inertial mass (the dynamical response), which slows the bar’s spin rather than its gravitational energy.”
“Our finding offers a fascinating perspective for constraining the nature of dark matter, as different models will change this inertial pull on the galactic bar,” said co-author and Ph.D. student Rimpei Chiba of the University of Oxford.
“Our discovery also poses a major problem for alternative gravity theories, as they predict no, or significantly too little, slowing of the bar because they lack dark matter in the halo.” Like other galaxies, the Milky Way is thought to be surrounded by a halo of dark matter that extends far beyond its visible edge.
Dark matter is invisible and unknown in its nature, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see. The Universe is thought to contain approximately five times as much dark matter as ordinary, visible matter.
Alternative gravity theories, such as modified Newtonian dynamics, reject the concept of dark matter in favor of explaining discrepancies by modifying Einstein’s theory of general relativity. The Milky Way galaxy is a barred spiral galaxy, with a thick bar of stars in the center and spiral arms extending through the disc beyond the bar. The bar rotates in the same way that the galaxy does.
The Royal Society, the Takenaka Scholarship Foundation, and the Science and Technology Facilities Council all provided funding for the study (STFC).
