Far out cosmology: Milky Way spin slows (Introduction)

by David Turell , Thursday, June 17, 2021, 15:18 (1254 days ago) @ David Turell

Finally measured:

https://www.ucl.ac.uk/news/2021/jun/dark-matter-slowing-spin-milky-ways-galactic-bar

"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 a new study by UCL and University of Oxford researchers.

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"In the study, published in the 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 – that is, they revolve around the galaxy at the same rate as the bar’s spin.

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

"The researchers found that the stars in the stream carry a chemical fingerprint – they are richer in heavier elements (called metals in astronomy), proving that they have travelled away from the galactic centre, 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 – made up of billions of stars and trillions of solar masses – had slowed down its spin by at least 24% since it first formed.

"Co-author Dr Ralph Schoenrich (UCL Mullard Space Science Laboratory) said: “Astrophysicists 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.

“'The 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."

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Comment: Full turning speed forever was never considered. More evidence for dark matter producing drag.