Darkish matter might sluggish the spin of the Milky Manner’s central bar of stars

Darkish matter could be a actual drag. The pull of that unidentified, invisible matter within the Milky Manner could also be slowing down the rotating bar of stars on the galaxy’s coronary heart.

Based mostly on a method that re-creates the historical past of the slowdown in a way akin to analyzing a tree’s rings, the bar’s pace has decreased by a minimum of 24 p.c because it fashioned billions of years in the past, researchers report within the August Month-to-month Notices of the Royal Astronomical Society.

That slowdown is “one other oblique however essential piece of proof that darkish matter is a factor, not only a conjecture, as a result of this will’t occur with out it,” says astrophysicist Martin Weinberg of the College of Massachusetts Amherst, who was not concerned with the examine.

Many spiral galaxies, together with the Milky Manner, comprise a central bar-shaped area densely filled with stars and surrounded by the galaxy’s pinwheeling arms. The bar additionally has some groupies: a crew of stars trapped by the bar’s gravitational affect. These stars orbit a gravitationally steady level positioned alongside the bar and farther from the galaxy’s middle, often called a Lagrange level (SN: 2/26/21). 

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If the bar’s rotation slows, it would develop in size, and the bar’s tagalongs can even transfer outward. As that occurs, that cohort of hangers-on will collect extra stars. In line with pc simulations of the method, these extra stars ought to organize themselves in layers on the surface of the group, says astrophysicist Ralph Schönrich of College Faculty London. The layers of stars imprint a report of the group’s progress. “It’s truly like a tree that you would be able to lower up in your personal galaxy,” he says.

Schönrich and astrophysicist Rimpei Chiba of the College of Oxford studied how the composition of stars within the group modified from its periphery to its deeper layers. Knowledge from the European Area Company’s Gaia spacecraft revealed that stars within the outer layers of the bar tended to be much less enriched in components heavier than helium than had been stars within the interior layers. That’s proof for the group of stars transferring outward, on account of the bar slowing, the researchers say. That’s as a result of stars within the middle of the galaxy — which might have glommed on to the group within the extra distant previous — are usually extra enriched in heavier components than these farther out.

The bar’s slowdown hints {that a} gravitational power is appearing on it, specifically, the pull of darkish matter within the galaxy. Regular matter alone wouldn’t be sufficient to scale back the bar’s pace. “If there is no such thing as a darkish matter, the bar is not going to decelerate,” Chiba says.

However the outcomes have drawn some skepticism. “Sadly, this isn’t but convincing to me,” says astrophysicist Isaac Shlosman of the College of Kentucky in Lexington. For instance, he doubts that the tree ring layering would actually happen. It’s “exhausting to imagine that that is the case in a sensible system” versus in a simplified pc simulation, he says.

Weinberg, then again, says that though the examine depends on quite a lot of assumptions, he suspects it’s appropriate. “It’s acquired the fitting scent.”

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