Milky Way's Dark Matter Halo is Slowing Down Galactic Bar's Spin, Astronomers Say | Astronomy

Milky Way's Dark Matter Halo is Slowing Down Galactic Bar's Spin, Astronomers Say | Astronomy

sci-news·2021-06-16 10:00

According to new research by astronomers from the University of Oxford and University College London, the spin of the bar of our Milky Way Galaxy — made up of billions of stars and trillions of solar masses — has slowed by more than 24% since its formation; galaxy models have long predicted such a slowdown by the postulated dark halo, but this is the first time it has been measured.An artists conception of the Milky Way Galaxy. Image credit: Pablo Carlos Budassi / CC BY-SA 4.0.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 Dr. Ralph Schoenrich, an astronomer in the Mullard Space Science Laboratory at University College London.Dr. Schoenrich and his colleague, University of Oxford Ph.D. student Rimpei Chiba, analyzed data from ESAs Gaia satellite on a large group of stars, the Hercules stream, which are in resonance with and gravitationally trapped by the Milky Ways bar.If the bars spin slows down, the stars in this stream would be expected to move further out in the Galaxy, keeping their orbital period matched to that of the bars spin.The astronomers found that these stars carry a chemical fingerprint — they are richer in heavier elements (called metals in astronomy), proving that they have traveled away from the Galactic center, where stars and star-forming gas are about 10 times as rich in metals compared to the outer Galaxy.Using these data, they inferred that the bar had slowed down its spin by at least 24% since it first formed.The counterweight slowing this spin must be dark matter, Dr. Schoenrich said.Until now, we have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter.Our research provides a new type of measurement of dark matter — not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bars spin.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, Chiba added.The finding also poses a major problem for alternative gravity theories — as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.The results were published in the Monthly Notices of the Royal Astronomical Society._____Rimpei Chiba & Ralph Schönrich. 2021. Tree-ring structure of Galactic bar resonance. MNRAS 505 (2): 2412-2426; doi: 10.1093/mnras/stab1094

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