Mapping the Galactic disk with red clump stars from LAMOST and Gaia II: 3D asymmetrical kinematics of mono-age populations in the disk between 6-14 kpc

Abstract

We perform analysis of the three-dimensional kinematics of Milky Way disk stars in mono-age populations. We focus on stars between Galactocentric distances of $R=6$ and 14 \\,kpc, selected from the combined LAMOST DR4 red clump giant stars and Gaia DR2 proper motion catalogue. We confirm the 3D asymmetrical motions of recent works, and we provide time tagging of the Galactic outer disk asymmetrical motions near the anticenter direction out to Galactocentric distances of 14\\,kpc. Radial Galactocentric motions reach values up to 10 km s$^{-1}$, depending on the age of the population, and present a north-south asymmetry in the region corresponding to density and velocity substructures that were sensitive to the perturbations in the early 6 \\,Gyr. After that time, the disk stars of this structure are becoming older and kinematically hotter and not sensitive to the possible perturbations, and we find it is a low $\\alpha$, metal rich, relatively younger population. With the quantitative analysis, we find stars both above and below the plane at $R\\gtrsim 9$ kpc exhibit bending mode motions of which the sensitive duration is around 8 \\,Gyr. Some possible scenarios for these asymmetries are discussed, including a fast rotating bar, spiral arms, minor mergers, sub-halos, warp dynamics, and streams. Although we cannot rule out other factors, for the current results, we speculate that the in-plane asymmetries might be mainly caused by gravitational attraction of overdensities in a spiral arm or monolithic collapse of isolated self-gravitating overdensities from out-of-equilibrium systems. Vertical motions might be dominated by bending and breathing modes induced by inner or external perturbers.