Is the Milky Way still breathing? RAVE-Gaia streaming motions
  We use data from the Radial Velocity Experiment (RAVE) and the Tycho-Gaia
astrometric solution catalogue (TGAS) to compute the velocity fields yielded by
the radial (VR), azimuthal (Vphi) and vertical (Vz) components of associated
Galactocentric velocity. We search in particular for variation in all three
velocity components with distance above and below the disc midplane, as well as
how each component of Vz (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 VR and Vz. Below the
plane we find the largest radial gradient to be dVR / dR = -7.01+- 0.61 km\s
kpc, in agreement with recent studies. Above the plane we find a similar
gradient with dVR / dR= -9.42+- 1.77 km\s kpc. By comparing our results with
previous studies, we find that the structure in Vz 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.
