The outer disk of our Milky Way Galaxy is warped and flared. Several mechanisms have been proposed to explain these phenomena, but none have reproduced both features. Recent work has demonstrated that the Milky Way’s stellar halo is tilted with respect to the disk plane, suggesting that at least some component of the dark matter halo may also be tilted. In a new modeling study, astronomers from the Harvard and Smithsonian’s Center for Astrophysics show that a dark halo tilted in the same direction as the stellar halo can induce a warp and flare in the Milky Way’s disk at the same amplitude and orientation as the data. The work bolsters current thinking about how our Galaxy evolved and may offer clues into some of the mysteries of dark matter.
From a great distance, our Milky Way Galaxy would look like a thin disk of stars that orbits once every few hundred million years around its central region, where hundreds of billions of stars provide the gravitational ‘glue’ to hold it all together. But this pull of gravity is much weaker in the Galaxy’s far outer disk. There, the hydrogen atoms making up most of the Milky Way’s gas disk are no longer confined to a thin plane, instead they give the disk an S-like, or warped, appearance. Image credit: Xiaodian Chen.
The Milky Way Galaxy is located inside a diffuse cloud called the stellar halo, which extends much farther out into the Universe.
In a previous work, Jiwon Jesse Han and colleagues deduced that the stellar halo is tilted and elliptical in shape, like a zeppelin or football.
Building on that, they assumed the same shape for the dark matter halo, the larger entity that encompasses everything in and around the Milky Way.
Dark matter makes up 80% of the Galaxy’s mass but is invisible because it doesn’t interact with light, so the shape of that halo must be inferred.
Using models to calculate the orbits of stars within a tilted, oblong dark matter halo, the astronomers found a near-perfect match to existing observations of a warped, flared galaxy.
“A tilted dark halo is actually fairly common in simulations, but no one had explored its effect on the Milky Way,” said Dr. Charlie Conroy, co-author of the study.
“It turns out that the tilt is an elegant way to explain both the magnitude and direction of our Galaxy’s wobbly disk.”
Astronomers had long surmised that the Milky Way formed due to a galactic collision; the new work further underscores that hypothesis.
“If the Galaxy was just evolving on its own, it would have had this nice, spherical halo, this nice, flat disk,” Han said.
“So the fact that the halo is tilted and has a football-like shape suggests that our Galaxy experienced a merger event, where two galaxies collide.”
Their calculation of the dark matter halo’s probable shape may also provide clues as to the properties and particle nature of dark matter itself, which remain unsolved mysteries in physics.
“The fact that the Galaxy is not spherical in our data implies that there is some limit to which dark matter can interact with itself,” Han said.
“Confidence in these findings might lead to better ways to cleverly study the unobservable dark matter that makes up most of the Universe.”
“This includes new ways to pick up on kinematic signatures of dark sub-halos, which are miniature dark matter halos zipping around the Galaxy.”
A paper on the findings was published in the journal Nature Astronomy.
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J.J. Han et al. A tilted dark halo origin of the Galactic disk warp and flare. Nat Astron, published online September 14, 2023; doi: 10.1038/s41550-023-02076-9
