Evidence of a dynamically evolving Galactic warp

Our Galaxy's disk is warped, and that warp is dynamic. Here, Poggio et al. measure the rate of precession of the Galaxy's warp using a large sample of giant stars. Rather than primordial, the rate of precession indicates that the Milky Way acquired its warp during a recent or even ongoing encounter with another galaxy. In a cosmological setting, the disk of a galaxy is expected to continuously experience gravitational torques and perturbations from a variety of sources, which can cause the disk to wobble, flare and warp(1,2). Specifically, the study of galactic warps and their dynamic nature could reveal key information on the formation history of galaxies and the mass distribution of their haloes. Our Milky Way presents a unique case study for galactic warps, thanks to detailed knowledge of its stellar distribution and kinematics. Using a simple model of how the warp's orientation is changing with time, here, we measure the precession rate of the Milky Way's warp using 12 million giant stars from Gaia Data Release 2(3), finding that it is precessing at 10.86 +/- 0.03 (statistical) +/- 3.20 (systematic) km s(-1) kpc(-1) in the direction of Galactic rotation, about one-third the angular rotation velocity at the Sun's position in the Galaxy. The direction and magnitude of the warp's precession rate favour 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.