The spin charge of the central bar of the Milky Manner is slowing down because of darkish matter

If it weren’t for a huge halo of dark matter enveloping our galaxy, the rate of rotation of our central bar would have to remain fairly constant. However, researchers recently found that it has slowed nearly 25% since it was created, a clear sign of the presence of dark matter.

How do you measure the spin rate of something that takes millions of years to make a single revolution?

The answer is chemistry. Star chemistry.

Stars near the center of the galaxy are much richer in “metals,” which astronomers use to refer to any element heavier than helium. Metals are much more lacking in the outskirts, however. So if you happen to come across a group of stars that are exceptionally rich in metals, they are most likely moving away from the center of the galaxy.

Such is the case with the Hercules Stream, a large group of stars observed with the Gaia satellite. The Hercules Current is a metal-rich star cluster that is relatively far from the galactic center. The Hercules steam has something else fascinating: it is caught in a gravitational dance with the central beam of the Milky Way.

Similar to how the Trojan asteroids of the solar system lead and follow Jupiter on its orbit around the sun, the Hercules vapor is captured by a unique combination of gravitational forces. Like the central bar, the Hercules Current vibrates.

The Hercules Current follows the movement of the central bar, and the stars of the Hercules Current have moved outwards to their current position in the last billion years.

The only way for the Hercules Current to move outward is to have the middle bar slow down. When the bar’s spin rate decreases, the Hercules stream must move outward to match the period.

In conclusion, a team of astronomers estimate that the bar’s spin rate has decreased by 24% since the Milky Way was formed, according to a new study published.

Co-author Dr. Ralph Schoenrich (UCL Mullard Space Science Laboratory) said, “Astrophysicists have long suspected that the rotating bar at the center of our galaxy is slowing down, but we have found the first evidence of this.

“The counterweight that slows this rotation must be dark matter. So far we could only infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from the visible matter.

“Our research offers a new way of measuring dark matter – not its gravitational energy, but its inertial mass (the dynamic response), which slows the rotation of the beam.”

Co-author and PhD student Rimpei Chiba of Oxford University said, “Our results offer a fascinating perspective to constrain the nature of dark matter as different models will alter this inertial force on the galactic bar.

“Our finding also poses a major problem for alternative theories of gravity – since they lack dark matter in the halo, they predict no or significantly too little slowing down of the bar.”

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