Vertical Structure of Galactic Disk Kinematics from LAMOST K Giants

Abstract

We examine the vertical structure of Galactic disk kinematics over a Galactocentric radial distance range of R  = 5–15 kpc and up to 3 kpc away from the Galactic plane, using the K-type giants surveyed by LAMOST. Based on robust measurements of three-dimensional velocity moments, a wobbly disk is detected in a phenomenological sense. An outflow dominates the radial motion of the inner disk, while in the outer disk there exist alternate outward and inward flows. The vertical bulk velocities is a combination of breathing and bending modes. A contraction-like breathing mode with amplitudes increasing with the distance to the plane and an upward bending mode dominate the vertical motion outside R 0 , and there are reversed breathing and bending modes at R  <  R 0 , with amplitudes much smaller than those outside R 0 . The mean azimuthal velocity decreases with the increasing distance to the plane, with gradients shallower for larger R . Stars in the south disk are rotating faster than stars in the north. The velocity ellipsoid orientation differs between different R : in the range of 5 < R < 9 kpc , the gradient of the tilt angle with respect to arctan ( Z / R ) decreases from ∼0.83 for the inner disk to ∼0.52 for the outer disk; within 9 < R < 15 kpc , the tilt of velocity ellipsoid deviates from vertical antisymmetry. A clear flaring signature is found for both north and south disks based on the observed vertical structures of velocity ellipsoid.