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

Gaia DR2 has revealed substructures in the phase space distribution of stars in the Milky Way. In particular, ridge like structures can be seen in the (R,V_phi) plane and asymmetric arches in (V_R, V_phi) plane. We show that the ridges are also clearly present when the (R,V_phi) plane is mapped by , ,, <|z|>,[Fe/H] and [alpha/Fe]. The last three maps suggest that stars along the ridges lie preferentially close to the Galactic midplane (|z|<0.2 kpc), and, have metallicity and alpha elemental abundance similar to that of the Sun. We show that phase mixing of disrupting spiral arms can generate both the ridges and the asymmetric arches. It also generates discrete groupings in orbital energy, and the ridges and arches are simply surfaces of constant energy. We identify 8 distinct ridges in the Gaia DR2 data, six of them have constant energy while two of them have constant angular momentum. The presence of ridges in 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.