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

Shourya Khanna, Sanjib Sharma, Thor Tepper-Garcia, Joss Bland-Hawthorn, Michael Hayden, Martin Asplund, Sven Buder, Boquan Chen, Gayandhi M. De Silva, Ken C. Freeman, Janez Kos, Jane Lin, Sarah L. Martell, Jeffrey D. Simpson, Dennis Stello, Yuan-Sen Ting, Daniel B. Zucker, Tomaz Zwitter

Published 2019-02-26Version 1

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 <z>, <V_z>, <V_R>, <[Fe/H]> and <[alpha/Fe]>. The 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 arches. It also generates discrete groupings in orbital energy - 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 have constant angular momentum. Given that the signature is strongest for stars close to the plane, the presence of ridges in <z> and <V_z> 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.