Something mysterious is tugging at the hem of our galaxy, the Milky Way, which is causing a “galactic warp” at its outer edges, according to a study published on Monday in Nature Astronomy.
The Milky Way contains an estimated 250 billion stars, which cluster in spiral arms that swirl around a central supermassive black hole. But astronomers have caught glimpses of a different dynamic occurring out in the far reaches of the Milky Way, where the galactic disk appears to become flared, wobbly, or otherwise distorted by a variety of potential phenomena.
To figure out what might be causing the Milky Way’s outer warping, a team led by Eloisa Poggio, an astrophysicist at the Italian National Institute for Astrophysics in Turin, measured the motions of 12 million giant stars observed by the European Space Agency’s Gaia space telescope.
“Our Milky Way presents a unique case study for galactic warps, thanks to detailed knowledge of its stellar distribution and kinematics,” Poggio’s team said in the study. “However, the cause and dynamic nature of the warp of our Galaxy have remained unclear due to a lack of kinematic constraints.”
In other words, scientists only began to model the Milky Way’s outer warping in detail within the past few years, due to advances in observational techniques. In 2019, for instance, a team created a captivating 3D map of the galactic warp based on measurements of 2,300 Cepheids variables, which are a special type of ultra-radiant stars (their results were published in Science )
Poggio and her colleagues have now built on these models thanks to the second major data dump from the Gaia satellite, which was released in 2018. Gaia has been constructing the most comprehensive catalog of the observable universe ever made since it was launched in 2013.
The sheer volume of observations enabled Poggio and her colleagues to measure the motions of millions of giant stars to trace out the kinematic signature of the galaxy’s warp. “We measured the speed of the warp by comparing the data with our models,” Poggio said in an ESA statement.
“Based on the obtained velocity, the warp would complete one rotation around the centre of the Milky Way in 600 to 700 million years,” she added. “That’s much faster than what we expected based on predictions from other models.”
While other teams have also used Gaia’s 2018 data to probe our galaxy’s edges, the new study aims to constrain “the possible mechanisms responsible for the warp of the Milky Way,” the team said. Suggested explanations for the warp have included longstanding torques emerging from mismatched axial motions of the disk and the galactic halo, or perhaps tidal forces from satellite galaxies of the Milky Way, which could gravitationally yank at the disk.
By modeling the motions of giant Gaia stars over time, Poggio’s team showed that the pull of a satellite galaxy is the most likely cause of the warp.
“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 study says.
This finding might corroborate other recent studies that reconstruct the Milky Way’s turbulent past with nearby galaxies, which can skim material from the galaxies edges or cause disturbances by colliding with it.
It will take more observations and research to confirm that the warp is mostly caused by satellite galaxies. “Nevertheless,” the team said, “our results suggest that external forces from interacting satellite galaxies are playing an important and ongoing role in shaping the outer disk of the Milky Way.”