Conventional wisdom has long held that the Milky Way was more or less a flat disk of stars and gas, with a bulge in the middle. But now astronomers from Macquarie University and the Chinese Academy of Sciences have created a more accurate 3D map of the galaxy and found that it's more warped and twisted than previously thought.
From our position in the wings of the Milky Way, it's hard to get an idea of exactly what shape the galaxy is. To develop a new map, the researchers looked to a group of stars known as classical Cepheids, which can be up to 20 times more massive than the Sun and 100,000 times brighter. That brightness tends to pulse in a regular pattern on a scale of days or weeks, and astronomers can use that to estimate their distance to within three to five percent.
For the new study the team examined 1,339 Cepheid stars, the data for which came from the Wide-field Infrared Survey Explorer (WISE). When they plotted these stars out on a 3D map, the researchers found that instead of being arranged in a flat disk they twist upwards at one end and down at the other, creating a kind of stretched out S shape. This seems to be true not just for the galaxy's stars but the disk of hydrogen gas as well.
"Somewhat to our surprise, we found that in 3D, our collection of 1,339 Cepheid stars and the Milky Way's gas disk follow each other closely," says Richard de Grijs, co-author of the paper. "This offers new insights into the formation of our home galaxy. Perhaps more importantly, in the Milky Way's outer regions, we found that the S-like stellar disk is warped in a progressively twisted spiral pattern."
The researchers attribute this pattern to the gravitational influence of the galactic center. Billions and billions of stars reside there – along with a huge helping of dark matter – and the gravity of that enormous mass holds the galaxy together. But the further out you go, the weaker that influence gets, so things can drift away a little. As the central mass spins, it creates strong torques, or rotational forces, on the outer regions, which gives it the twisted shape.
While it isn't unprecedented for a galaxy to have this shape, it is still relatively rare, with only a dozen or so other examples known to exhibit similar twists. The team studied those other galaxies to better understand our own.
"Combining our results with those other observations, we concluded that the Milky Way's warped spiral pattern is most likely caused by torques—or rotational forcing—by the massive inner disk," says Dr. Liu Chao, co-author of the paper.
The research was published in the journal Nature Astronomy.
Source: Macquarie University, Chinese Academy of Sciences via Science Daily