Data from ESA’s Gaia star-mapping satellite show that the warped disk of our Milky Way Galaxy precesses, or wobbles, similarly to the motion of a spinning top. The warp moves around the Galactic center faster than previously expected, completing one rotation in 600 to 700 million years. That’s however, still slower than the speed at which the stars in the disk orbit the Galactic center. The Sun, for example, completes one rotation in about 220 million years. The speed of the warp’s precession led an international team of astronomers to believe that it must be caused by a recent or ongoing collision with another, smaller, galaxy.
The structure of our Milky Way Galaxy with its warped Galactic disk, where the majority of its hundreds of billions of stars reside. Data from ESA’s Gaia spacecraft recently proved that the disk’s warp is precessing, essentially moving around similarly to a wobbling spinning top. The speed of the warp’s rotation is so high that it must have been caused by a rather powerful event, astronomers believe, perhaps an ongoing collision with another, smaller, galaxy which sends ripples through the disk like a rock thrown into water. Image credit: Stefan Payne-Wardenaar / NASA / JPL-Caltech / ESA.
Dr. Eloisa Poggio, an astronomer at the Turin Astrophysical Observatory, and colleagues measured the precession rate of the Milky Way’s warp using 12 million giant stars from the second Gaia data release.
They found that it is precessing at 10.86 ± 0.03 (statistical) ± 3.20 (systematic) km/s*kpc in the direction of Galactic rotation.
“We measured the speed of the warp by comparing the data with our models,” Dr. Poggio said.
“Based on the obtained velocity, the warp would complete one rotation around the center of the Milky Way in 600 to 700 million years.”
“That’s much faster than what we expected based on predictions from other models, such as those looking at the effects of the non-spherical halo.”
The direction and magnitude of the warp’s precession rate favor the scenario that the warp is the result of a recent or ongoing encounter with a satellite galaxy, rather than the relic of the ancient assembly history of the Galaxy.
The astronomers do not yet know which galaxy might be causing the ripple nor when the collision started.
One of the contenders is the Sagittarius dwarf galaxy, a small satellite of the Milky Way that is leaving a stream of stars behind as an effect of our Galaxy’s gravitational tug and is believed to have burst through the Galactic disk several times in the past.
Astronomers think that Sagittarius will be gradually absorbed by the Milky Way.
“With Gaia, for the first time, we have a large amount of data on a vast amount stars, the motion of which is measured so precisely that we can try to understand the large scale motions of the galaxy and model its formation history,” said ESA’s Gaia deputy project scientist Dr. Jos de Bruijne.
“This is something unique. This really is the Gaia revolution.”
As impressive as the warp and its precession appear on the galactic scale, the scientists reassure us that it has no noticeable effects on life on our planet.
“The Sun is at the distance of 26,000 light-years from the Galactic center where the amplitude of the warp is very small,” Dr. Poggio said.
“Our measurements were mostly dedicated to the outer parts of the Galactic disk, out to 52,000 light-years from the Galactic center and beyond.”
The results appear in the journal Nature Astronomy.
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E. Poggio et al. Evidence of a dynamically evolving Galactic warp. Nat Astron, published online March 2, 2020; doi: 10.1038/s41550-020-1017-3
