Astronomers Discover the Unexpected Warped Shape of the Milky Way

Earth sits about 25,000 light years away from the center of the Milky Way, so when it comes to our place in our galaxy, we’re hanging out in the suburbs. Our position inside the Milky Way has made it very difficult for astronomers to count the number of its stars and determine its shape. Until now, it was accepted that our galaxy was shaped like other spiral galaxies: flat like a pancake. A new map, however, reveals that the truth is much more twisted.

In a study published Monday in Nature Astronomy a team of Chinese scientists announced that the Milky Way is actually shaped into a “twisted spiral pattern” that looks especially S-like in its outer regions. While a distant view of the Milky Way might make it seem like one is examining a thin disc of stars, far away from its gravity-heavy inner regions are warped edges of hydrogen atoms, no longer confined to a flat plane.

The Milky Way has hundreds of billions of stars, a supermassive black hole center, and a huge mass of dark matter — it’s a behemoth held together by gravitational “glue.” The team behind the new paper added to this picture through the creation of a Classical Cepheids database. These are young stars that are 20 times as massive as the sun and up to 100,000 times as bright. Because these stars are so bright, they served as pinpoints on a new 3-dimensional map, calibrating the shape of the galaxy.

galaxy map
Top: 3D distribution of the Cepheids in the Milky Way's warped disk. Lower: Precession of the warp's line of nodes with Galactocentric radius.

“It is notoriously difficult to determine the distances from the sun to parts of the Milky Way’s outer gas disk without having a clear idea of what that disk actually looks like,” lead author Xiaodian Chen, Ph.D., said Monday. “However, we recently published a new catalogue of well-behaved variable stars known as classical Cepheids, for which distances as accurate as 3 to 5 percent can be determined.”

From this data, using the reliable Cepheids as reference points, Chen and his team extrapolated that the warped spiral of the Milky Way’s outer ridges is linked to rotational forces spinning out from its massive inner disk. This torque warps the outer regions, causing the galaxy to take on a progressively twisted spiral pattern — the farther stars are from its center, the more twisted its disk of stars becomes.

It all makes for a rare phenomenon that astronomers have seen in other galaxies — but not ours. This research, senior researcher and co-author Licai Deng, Ph.D., says, “provides a crucial updated map for studies of our galaxy’s stellar motions and the origins of the Milky Way’s disk.” The birth of the Milky Way was a billion-year process that scientists are still working towards understanding, and that picture just got a little clearer

Abstract: The Milky Way’s neutral hydrogen (H I) disk is warped and flared1,2. However, a dearth of accurate H I-based distances has thus far prevented the development of an accurate Galactic Disk model. Moreover, the extent to which our Galaxy’s stellar and gas disk morphologies are mutually consistent is also unclear. Classical Cepheids, primary distance indicators with distance accuracies of 3–5% (ref. 3), offer a unique opportunity to develop an intuitive and accurate three-dimensional picture. Here, we establish a robust Galactic Disk model based on 1,339 classical Cepheids. We provide strong evidence that the warp’s line of nodes is not oriented in the Galactic Centre–Sun direction. Instead, it subtends a mean angle of 17.5° ± 1° (formal) ± 3° (systematic) and exhibits a leading spiral pattern. Our Galaxy thus follows Briggs’ rule for spiral galaxies4, which suggests that the origin of the warp is associated with torques forced by the massive inner disk5. The stellar disk traced by Cepheids follows the gas disk in terms of their amplitudes; the stellar disk extends to at least 20 kpc (refs. 6,7). This morphology provides a crucial, updated map for studies of the kinematics and archaeology of the Galactic Disk.

Media via Xiaodian Chen, Jet Propulsion Laboratory, Chinese Academy of Sciences Headquarters 
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