Altered Shape of the Milky Way: The Role of Dark Matter and Galactic Interactions According to Harvard Astronomers

Harvard-based astronomers propose that the anomalous curvature of the Milky Way can be attributed to an uneven halo of dark matter. This idea lends credence to earlier theories suggesting that the galaxy has undergone a collision in the past and provides additional understanding about the elusive nature of dark matter. Acknowledgment: Stefan Payne-Wardenaar; Magellanic Clouds: Robert Gendler/ESO

The findings of these astronomers reinforce theories regarding the evolutionary path of our galaxy.

Contrary to popular representations, the Milky Way is not a flat, rotating disk of dust, gas, and stars. A more accurate portrayal would reveal a distinct curvature, akin to distorting a vinyl record.

For a long time, scientists were aware, based on observational data, that the Milky Way has a twisted shape and flared edges, much like the hem of a skirt. However, the underlying reason for this was not understood.

Contributions of Harvard Astronomers

Researchers at the Center for Astrophysics | Harvard and Smithsonian (CfA) have conducted the first comprehensive calculations to elucidate this enigma. Their compelling evidence indicates that an irregular halo of dark matter surrounds the Milky Way. This research not only fortifies existing theories about the galaxy’s formation but may also offer significant clues about the unidentified properties of dark matter.

In a representation of this phenomenon, NASA’s Hubble Space Telescope captured images of the Galaxy ESO, which like the Milky Way, exhibits a warped and flared galactic disk. This discovery reveals details about its distorted dusty disc and highlights how the collision of galaxies can lead to the formation of new stars. Acknowledgment: NASA/Space Telescope Science Institute

The primary calculations were spearheaded by Jiwon Jesse Han, a student at the Griffin Graduate School of Arts and Sciences and associated with the CfA. The research, which was published in the journal Nature Astronomy, also featured co-authors Charlie Conroy and Lars Hernquist, both of whom are faculty members at the CfA and the Department of Astronomy.

The Stellar Halo and the Enigma of Dark Matter

The Milky Way resides within a dispersed cloud known as the stellar halo, extending far beyond the galaxy itself. In a seminal study published last year, Harvard researchers posited that this halo is not spherical but is inclined and elliptical, resembling a zeppelin or a football.

Building upon this, the team hypothesized that the dark matter halo—which encompasses all matter within and surrounding the Milky Way—would have a similar shape. Given that dark matter constitutes 80% of the galaxy’s mass and remains invisible due to its non-interaction with light, the shape of the halo is implied rather than directly observed. Utilizing models to track the stellar orbits within this tilted, elongated dark matter halo, the team’s findings closely aligned with existing observations of a skewed, flared galaxy.

Implications for Future Research

“A skewed dark matter halo is actually quite prevalent in simulations, but its influence on the Milky Way had not been previously considered,” stated Conroy. “It appears that the tilt is a refined explanation for the galaxy’s oscillating disk, both in its amplitude and orientation.”

The calculations further strengthen the long-standing supposition that the Milky Way’s formation was a result of a past galactic collision. “A spherical halo and a flat disk would be expected if the galaxy evolved in isolation,” Han noted. “The tilted and football-shaped halo suggests a collisional history for our galaxy.”

The team’s estimation of the likely shape of the dark matter halo could also furnish hints about dark matter’s properties and its particle nature, topics still unresolved in the field of physics. “The non-spherical nature of our data suggests some constraints on how dark matter can interact with itself,” explained Han.

The credibility of these findings may pave the way for innovative techniques to study the unobservable dark matter that constitutes the majority of the universe, including new methods to detect the kinematic characteristics of dark sub-halos—smaller conglomerates of dark matter orbiting the galaxy.

Reference: “A tilted dark halo origin of the Galactic disk warp and flare” by Jiwon Jesse Han, Charlie Conroy, and Lars Hernquist, dated 14 September 2023, published in Nature Astronomy.
DOI: 10.1038/s41550-023-02076-9

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More about Dark Matter and Milky Way

• Harvard Center for Astrophysics
• Nature Astronomy Journal
• NASA’s Hubble Space Telescope
• Dark Matter: An Overview
• Galactic Collisions and Evolution
• Stellar Halo: An Introduction