The GALAH survey and Gaia DR2: Linking ridges, arches and vertical waves in the kinematics of the Milky Way.

Gaia DR2 has revealed new small-scale and large-scale patterns in the phase space distribution of stars in the Milky Way. In particular, ridge-like structures can be seen in the (R,Vphi) plane and asymmetric arch-like structures in the (Vphi,VR) plane. We show that the ridges are also clearly present when the third dimension of the (R,Vphi) plane is represented by , , , and . The maps suggest that stars along the ridges lie preferentially close to the Galactic midplane (|z| and suggests a coupling between planar and vertical directions. We demonstrate using N-body simulations that such coupling can be generated both in isolated discs and in discs perturbed by an orbiting satellite like the Sagittarius dwarf galaxy.