The Milky Way is embedded in a tilted dark matter halo, research suggests

The outer disk of our Milky Way Galaxy is burning away. Several mechanisms have been proposed to explain these phenomena, but none reproduce both features. Recent work has demonstrated that the stellar halo of the Milky Way is tilted with respect to the disk plane, suggesting that at least some components of the dark matter halo may be tilted. In a new modeling study, astronomers from Harvard and the Smithsonian’s Center for Astrophysics show that a dark halo tilted in the same direction as the stellar halo can induce a warp and a stretch in the Milky Way’s disk. The work advances current thinking about how our galaxy formed and may provide clues to some of the mysteries of dark matter.

From afar, our Milky Way Galaxy looks like a thin disk of stars orbiting its central region every few hundred million years, where hundreds of billions of stars provide the gravitational pull. But this gravitational force is very weak in the outer disk of the galaxy. There, the hydrogen atoms that make up the bulk of the Milky Way’s gas disk are no longer confined to a thin plane, but instead give the disk an S-like or skewed appearance. Photo credit: Xiaodian Chen.

The Milky Way galaxy is located within a diffuse cloud known as the stellar halo, which extends out into the universe.

In earlier work, Jiwon Jesse Han and colleagues deducted The star’s halo is elliptical in shape, tilted like a zeppelin or a football.

Based on that, they derived the same shape for the dark matter halo, the larger entity that encompasses the Milky Way and everything around it.

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Dark matter makes up 80% of the galaxy’s mass, but it is invisible because it does not interact with light, so the shape of that halo must be inferred.

Using models to calculate the orbits of stars within the halo of a tilted, elliptical dark matter, astronomers found an almost perfect match with current observations of a collapsed, expanding galaxy.

„A tilted dark halo is actually very common in simulations, but no one has investigated its effect on the Milky Way,” said Dr Charlie Conroy, co-author of the study.

„The tilt is an elegant way to explain both the size and direction of our galaxy’s wobble disk.”

Astronomers have long speculated that the Milky Way was formed as a result of interstellar collisions; New work further underscores that hypothesis.

„If the galaxy formed on its own, it would have this nice, spherical halo, this nice, flat disk,” Hahn said.

„So the fact that the halo is tilted and shaped like a football suggests that our galaxy experienced a merger event where two galaxies collided.”

Calculation of the possible shape of the dark matter halo may provide clues about the properties and particle nature of dark matter, which remain unsolved mysteries in physics.

„The fact that the galaxy is not spherical in our data suggests that dark matter has some limits to interacting with itself,” Hahn said.

„Confidence in these findings will lead to better ways of intelligently probing the elusive dark matter that makes up most of the universe.”

„This includes new ways to pick up the kinematic signatures of dark sub-haloes, which are miniature dark matter haloes orbiting galaxies.”

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A Paper The findings were published in the journal Natural Astronomy.

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JJ Han and many others. Oblique dark halo appearance of galactic disk warp and flare. Nat Astron, published online September 14, 2023; doi: 10.1038/s41550-023-02076-9

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