The Great Wave: Evidence of a large-scale vertical corrugation propagating outwards in the Galactic disc

Abstract

We analyse the three-dimensional structure and kinematics of two samples of young stars in the Galactic disc, containing respectively young giants ($\sim$17000 stars out to heliocentric distances of $\sim$7 kpc) and classical Cepheids ($\sim$3400 stars out to heliocentric distances of $\sim$15 kpc). The vertical structure of the two samples exhibit a consistent shape of the Milky Way's warp, whose amplitude reaches $\sim$700 pc at a Galactocentric radius R $\sim$ 14 kpc. Moreover, both samples show evidence of a large-scale vertical corrugation on top of the warp with a vertical height of $\sim$150-200 pc, extending over a large portion of the Galactic disc between Galactocentric radii $R\sim$ 10-12 kpc in the third Galactic quadrant (galactic longitudes $180^{\circ }<l<270^{\circ }$) and $\sim$12-14 kpc in the second Galactic quadrant ($90^{\circ }<l<180^{\circ }$). Its total length is at least 10 kpc and can possibly reach $\sim$ 20 kpc with the Cepheid sample. The stars in the corrugation exhibit both radial and vertical systematic motions, with Galactocentric radial velocities towards the outer disc of about 10-15 km/s. 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 $\pi /2$), 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, a $\sim$2.7 kpc filament of dense gas clouds close to the Sun, and exhibits a larger coverage of the Galactic disc.

24 pages, 23 figures, and 2 tables. The revised version of the paper includes an improved discussion on waves in the Galactic disc, a comparison with the spiral structure and the Radcliffe wave, as well as the presentation of a simple toy model to interpret the data. Accepted for publication in A&A

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