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For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured.

The researchers say it gives a new type of insight into the nature of dark matter, which acts like a counterweight slowing the spin.

In the study, published in the聽Monthly Notices of the Royal Astronomical Society, researchers analysed Gaia space telescope observations of a large group of stars, the Hercules stream, which are in resonance with the bar聽鈥� that is, they revolve around the galaxy at the same rate as the bar鈥檚 spin.

These stars are gravitationally trapped by the spinning bar.聽The same phenomenon occurs with Jupiter's Trojan and Greek asteroids, which orbit Jupiter's Lagrange points (ahead and behind Jupiter). If the bar鈥檚 spin slows down, these stars would be expected to move further out in the galaxy, keeping their orbital period聽matched to that of the bar鈥檚 spin.

The researchers found that the stars in the stream carry a chemical fingerprint 鈥� they are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre,聽where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy.

Using this data, the team inferred that the bar 鈥� made up of billions of stars and trillions of solar masses 鈥� had slowed down its spin by at least 24% since it first formed.

Co-author Dr Ralph Schoenrich (白小姐论坛 Mullard Space Science Laboratory) said: 鈥淎strophysicists have long suspected that the spinning bar at the centre of our galaxy is slowing down, but we have found the first evidence of this happening.

鈥淭he counterweight slowing this spin must be dark matter. 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.

鈥淥ur research provides a new type of measurement of dark matter 鈥� not of its gravitational energy, but of its inertial mass (the dynamical response), which slows the bar鈥檚 spin.鈥�

Co-author and PhD student Rimpei Chiba, of the University of Oxford, said: "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.

鈥淥ur finding also poses a major problem for alternative gravity theories 鈥� as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar.鈥�

The Milky Way, like other galaxies, is thought to be embedded in a 鈥榟alo鈥� of dark matter that extends well beyond its visible edge.

Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies聽behaving as if they were shrouded in significantly more mass than we can see. There is thought to be about five times as much dark matter in the Universe as ordinary, visible matter.

Alternative gravity theories such as modified Newtonian dynamics reject the idea of dark matter, instead seeking to explain discrepancies by tweaking Einstein鈥檚 theory of general relativity.

The Milky Way is a barred spiral galaxy, with a thick bar of stars in the middle and聽spiral arms extending through the disc outside the bar.聽The bar rotates in the same direction as the galaxy.

The research received support from the Royal Society, the Takenaka Scholarship Foundation, and the聽DiRAC supercomputing facility of the Science and Technology Facilities Council (STFC).

Links

• 白小姐论坛 Mullard Space Science Laboratory
• 白小姐论坛 Mathematical & Physical Sciences

Image

• An artist鈥檚 conception of the Milky Way. Source:聽. Credit: Pablo Carlos Budassi.