Velocity Shear of the Thick Disk from SPM3 Proper Motions at the South Galactic Pole

The kinematical properties of the Galactic thick disk are studied using absolute proper motions from the Third Yale/San Juan Southern Proper Motion Catalog and Two Micron All Sky Survey near-infrared photometry for a sample of ∼1200 red giants in the direction of the south Galactic pole. The photometrically selected sample is dominated by thick-disk stars, as indicated by the number-density distribution that varies with distance from the Galactic plane as a single-valued exponential over the range 1 kpc < z < 4 kpc. The inferred scale height of the thick disk is 0.783 ± 0.048 kpc. The kinematics of the sample are also consistent with disklike motion. The U-velocity component is roughly constant, reflecting the Sun's peculiar motion, while a considerable shear is seen in the mean rotational velocity, V. The V-velocity profile's dependence on z is linear, with a gradient of dV/dz = -30 ± 3 km s-1 kpc-1. The velocity dispersions in both U and V show a lesser gradient of about 9 ± 3 km s-1 kpc-1. We demonstrate that the derived velocity and velocity-dispersion profiles are consistent with the assumptions of dynamical equilibrium and reasonable models of the overall Galactic potential.