Astronomers have known for decades that our galaxy is warped. Now, they can follow the warp’s rotation as it travels around the galactic center.

The warped Milky Way

Stefan Payne-Wardenaar; Magellanic Clouds: Robert Gendler / ESO

Pictures can be deceiving.

Images of galaxies near and far show grand-design spirals, fuzzy ellipticals, and mussed-up irregulars as if frozen in time. But in reality, these galaxies are constantly in motion.

The Sun, for example, is whizzing around our galactic center at some 230 kilometers per second (515,000 mph). Indeed, most stars in our galaxy are moving largely in circles around the center — but not all of them.

Farther from the center, our galaxy is warped, with one side swung upward and the other downward. The stars in the outskirts follow suit, riding like boats up and down the wave. Scientists recently measured those motions, using the European Space Agency’s Gaia satellite to gauge the incredible speed with which the warp whirls around the Milky Way. The results appear in the March 2nd Nature Astronomy.

Galactic Warp

Scientists have long known that, while the inner 20,000 light-years of the Milky Way are flat as a very thin pancake, the outer edges warp. The Sun’s orbit has it circling the center right outside the flat region. So if you look out into the night sky in the direction opposite the galactic center in Sagittarius, you’re looking toward the outer reaches of the galaxy, where the warp starts to take effect.

The warp’s effect is subtle — in fact, astronomers first noticed it in the signature of neutral hydrogen gas in the galactic outskirts, which could be better tracked at greater distances than stars could.

Now, though, we have the Gaia satellite that’s mapping the positions and motions of more than a billion Milky Way stars. Last year, scientists used Gaia data on Cepheid variable stars to precisely chart the warp’s shape. Now, Eloisa Poggio (National Institute of Astrophysics, Italy) and colleagues have again turned to Gaia to see how that warp precesses, or turns, around the center.

Millions of Giants

The researchers started by selecting more than 12.6 million giant stars from the Gaia catalog, looking at how they move upward or downward with respect to the galactic plane.

Even though the stars’ motions were measured only over a short period of time, that data gave the researchers enough information to predict the future motions of the warp itself.

“It’s like having a car and trying to measure the velocity and direction of travel of this car over a very short period of time and then, based on those values, trying to model the past and future trajectory of the car,” adds coauthor Ronald Drimmel (Turin Astrophysical Observatory, Italy). “Similarly, by measuring the apparent motions of millions of stars across the sky, we can model large-scale processes such as the motion of the warp.”

Data from the European Space Agency's star-observer Gaia mission show that the warp in the Milky Way's disk is precessing, essentially moving around similarly to a wobbling spinning top. The team collected the data from giant stars out to 52,000 light-years from the galaxy's center.

Stefan Payne-Wardenaar; Inset: NASA / JPL-Caltech; Layout: ESA

Recently Walloped

"This is an interesting paper making use of the beautiful Gaia DR2 data with a simple model," says Chervin Laporte (Kavli Institute of the Physics and Mathematics of the Universe, Japan), who was not involved in the study. "By inferring the precession rate of the warp in the volume covered by Gaia, the authors hope to bring light on the 'origin' of the warp in the galaxy. . . . What is really interesting to me in [this] paper is the measurement of the precession rate in the Milky Way."

The researchers find that the warp is turning prograde, in the direction of the galaxy’s rotation, at a speed of about 10 km/s per kiloparsec away from the galactic center. That means that out at the Sun’s location, at 26,000 light-years (8 kiloparsecs), the warp is precessing at 80 km/s — so our star is fast outpacing the wave it rides. While the Sun takes some 220 million years to circle the galaxy, the warp takes much longer, about 600 or 700 million years.

Yet the warp is turning considerably faster than expected by some theories. Despite some uncertainty in the measurement of the warp’s rate of turning, the results are certain enough to indicate that the warp hasn’t always been there. A recent interaction with a dwarf galaxy — suggested by several other lines of evidence — could have temporarily whacked our galaxy out of a flatter state of existence.