Detection of a radial velocity gradient in the extended local disc with
  RAVE
  Using a sample of 213,713 stars from the Radial Velocity Experiment (RAVE)
survey, limited to a distance of 2 kpc from the Sun and to |z|<1 kpc, we report
the detection of a velocity gradient of disc stars in the fourth quadrant,
directed radially from the Galactic centre. In the direction of the Galactic
centre, we apply a simple method independent of stellar proper motions and of
Galactic parameters to assess the existence of this gradient in the RAVE data.
This velocity gradient corresponds to |K+C| < 3 km/s/kpc, where K and C are the
Oort constants measuring the local divergence and radial shear of the velocity
field, respectively. In order to illustrate the effect, assuming a zero radial
velocity of the Local Standard of Rest we then reconstruct the two-dimensional
Galactocentric velocity maps using two different sets of proper motions and
photometric distances based either on isochrone fitting or on K-band
magnitudes, and considering two sets of values for the Galactocentric radius of
the Sun and local circular speed. Further observational confirmation of our
finding with line-of-sight velocities of stars at low latitudes, together with
further modelling, should help constrain the non-axisymmetric components of the
Galactic potential, including the bar, the spiral arms and possibly the
ellipticity of the dark halo.
