2018MNRAS.475.2679C

We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia astrometric solution (TGAS) catalogue to compute the velocity fields yielded by the radial (V_R), azimuthal (V_ph),and vertical (V_z) components of associated Galactocentric velocity. We search in particular for variation in all three velocity components with distance above and below the disc mid-plane, as well as how each component of V_z (line-of-sight and tangential velocity projections) modifies the obtained vertical structure. To study the dependence of velocity on proper motion and distance, we use two main samples: a RAVE sample including proper motions from the Tycho-2, PPMXL, and UCAC4 catalogues, and a RAVE-TGAS sample with inferred distances and proper motions from the TGAS and UCAC5 catalogues. In both samples, we identify asymmetries in V_R and V_z. Below the plane, we find the largest radial gradient to be ∂V_R/ ∂R = -7.01 ± 0.61 km s^–1 kpc^–1, in agreement with recent studies. Above the plane, we find a similar gradient with ∂V_R/ ∂R = -9.42 ± 1.77 km s^–1 kpc^–1. By comparing our results with previous studies, we find that the structure in V_z is strongly dependent on the adopted proper motions. Using the Galaxia Milky Way model, we demonstrate that distance uncertainties can create artificial wave-like patterns. In contrast to previous suggestions of a breathing mode seen in RAVE data, our results support a combination of bending and breathing modes, likely generated by a combination of external or internal and external mechanisms.