The great wave

We analysed the three-dimensional structure and kinematics of two samples of young stars in the Galactic disc, containing young giants (~17 000 stars out to heliocentric distances of ~7 kpc) and classical Cepheids (~3400 stars out to heliocentric distances of ~15 kpc), respectively. The vertical structure of the two samples exhibit a consistent shape of the Milky Way’s warp, whose amplitude reaches ~700 pc at a galactocentric radius R ~ 14 kpc. Moreover, both samples show evidence of a large-scale vertical corrugation on top of the warp with a vertical height of ~150-200 pc, extending over a large portion of the Galactic disc between galactocentric radii of R ~ 10-12 kpc in the third Galactic quadrant (galactic longitudes of 180° < l < 270°) and ~12-14 kpc in the second Galactic quadrant (90° < l < 180°). Its total length is at least 10 kpc and could possibly reach ~20 kpc with respect to the Cepheid sample. The stars in the corrugation exhibit both radial and vertical systematic motions, with galactocentric radial velocities of about 10-15 km/s directed towards the outer disc. In the vertical motions, once the warp signature is subtracted, the residuals show a large-scale feature of systematically positive vertical velocities, which is shifted to slightly larger galactocentric radii with respect to the spatial vertical corrugation (with a phase difference of roughly π/2), indicating an oscillatory behaviour. A comparison of the observed shift with a simple toy model suggests that the corrugation can be interpreted as a wave propagating towards the outer disc. The wave mapped in this work is located at larger heliocentric distances compared to the Radcliffe wave, which is a ~2.7 kpc filament of dense gas clouds close to the Sun, and exhibits a larger coverage of the Galactic disc.