Vertical Structure of Galactic Disk Kinematics from LAMOST K Giants

Abstract

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.