Astronomers find an explanation for Milky Way’s warp

The results bolster hypothesis of how galaxy evolved.

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The Milky Way Galaxy’s outer disk is distorted and flared. To explain these events, several mechanisms have been put up, but none have been able to reproduce both aspects quantitatively.

The first calculations to adequately explain this anomaly have now been completed by Harvard astronomers at the Center for Astrophysics | Harvard and Smithsonian (CfA), with strong evidence pointing to the Milky Way’s encirclement in an aberrant halo of dark matter. The research supports current theories about how the galaxy developed and could provide answers to some of the riddles surrounding dark matter.

The Milky Way is located within a diffuse cloud called the stellar halo. This halo extends much farther out into the universe.

Previous studies deduced that the stellar halo is tilted and elliptical, like a zeppelin or football. Building on that, scientists in this new study assumed the same shape for the dark matter halo, which encompasses everything in and around the Milky Way.

The geometry of that halo must be inferred because dark matter, which makes up 80% of the galaxy’s mass, is invisible because it doesn’t interact with light. The team discovered a nearly perfect match to previous findings of a warped, flared galaxy using models to calculate the orbits of stars within a tilted, oblong dark matter halo.

Co-authors Charlie Conroy and Lars Hernquist said, “A tilted dark halo is fairly common in simulations, but no one had explored its effect on the Milky Way. It turns out that the tilt is an elegant way to explain both the magnitude and direction of our galaxy’s wobbly disk.”

The team’s work supports the theory that the Milky Way evolved due to a galactic collision, which scientists have long postulated.

Jiwon Jesse Han, a Griffin Graduate School of Arts and Sciences student affiliated with the CfA, said“If the galaxy was evolving on its own, it would have had this nice, spherical halo, this nice, flat disk. 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, which remain 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.”

If these results are confirmed, it may be possible to develop cleverer methods for studying the invisible dark matter that makes up most of the cosmos. This covers new techniques for detecting the kinematic signatures of dark sub-halos—miniature dark matter halos whirling around the galaxy.

Journal Reference:

  1. Han, J.J., Conroy, C. & Hernquist, L. A tilted dark halo origin of the Galactic disk warp and flare. Nat Astron (2023). DOI: 10.1038/s41550-023-02076-9

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