Harvard astronomers explain the mysterious warping of the Milky Way

Astronomers’ findings support the hypothesis of how our galaxy evolved.

the milky way It is often depicted as a flat, rotating disk of dust, gas, and stars. But if you could zoom out and take a surround picture, it actually has a distinct warp to it – as if you were trying to twist and bend a vinyl LP.

Although scientists have long known from observational data that the Milky Way is distorted and that its edges flare like a skirt, no one has been able to explain why.

Discoveries of Harvard astronomers

And now Harvard astronomers at the Center for Astrophysics | Harvard and Smithsonian (CFA) performed the first calculations that fully explain this phenomenon, with compelling evidence pointing to the Milky Way being enveloped in an irregular halo of dark matter. This work also advances current thinking about how the galaxy evolves, and may provide clues to some of the mysteries of dark matter.

The galactic disk of the Milky Way is twisted and glowing, similar to the Galaxy ESO pictured here. NASA’s Hubble Space Telescope imaged this unusual galaxy, revealing remarkable details of its dusty, warped disk and showing how colliding galaxies give birth to new stars. The dust and spiral arms of ordinary spiral galaxies, like our own Milky Way, appear flat when viewed from the edge. Image source: NASA/Space Telescope Science Institute

The new calculations were led by Jiwon Jesse Han, a student in the CfA-affiliated Griffin Graduate School of Arts and Sciences. Published in the magazine Nature astronomyThe work includes co-authors Charlie Conroy and Lars Hernquist, both faculty members at CfA and in the Department of Astronomy.

Stellar corona and dark matter

Our galaxy lies within a diffuse cloud called the stellar corona, which extends farther into the universe. In groundbreaking work published last year, the Harvard team concluded that the stellar corona is tilted and elliptical, like a Zeppelin or a football.

Accordingly, the team took the same shape for the dark matter halo, the larger entity that includes everything in and around the Milky Way. 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 deduced. Using models to calculate the orbits of stars inside a rectangular, tilted halo of dark matter, the team found a near-perfect match with current observations of a warped, glowing galaxy.

Monuments and visions

“The tilted dark corona is actually fairly common in simulations, but no one has explored its effect on the Milky Way,” Conroy said. “It turns out that tilt is a great way to explain the size and direction of our galaxy’s oscillating disk.”

Scientists have long believed that the Milky Way formed as a result of a galactic collision; The work of astronomers confirms this hypothesis.

“If the galaxy was evolving on its own, it would have this beautiful spherical halo, this beautiful flat disk,” Hahn 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.”

“It turns out that tilt is a great way to explain the size and direction of our galaxy’s oscillating disk.”
— Charlie Conroy, Department of Astronomy

Their calculations of the possible shape of the dark matter halo may also provide clues about the properties and nature of dark matter particles themselves, which remain unsolved mysteries in physics. “The fact that the galaxy is not spherical in our data suggests that there is some limit to which dark matter can interact with itself,” Hahn explained.

Confidence in these results may lead to better ways to intelligently study the unobservable dark matter that makes up most of the universe. This includes new ways to capture the kinematic signatures of dark subhalos, which are small halos of dark matter wandering around the galaxy.

Reference: “The origin of the oblique dark halo and flare of the galactic disk” by Jeon Jesse Han, Charlie Conroy, and Lars Hernequist, September 14, 2023, Nature astronomy.
doi: 10.1038/s41550-023-02076-9