Kinematics analysis of the transient Milky Way warp using RGB stars from Gaia DR2

Abstract

The perturbations to the Milky Way cause the disk to have bending or breathing wave, flare, and warp. Both spatial analyses from stellar and gas show the Milky Way warp can be approached with sinusoidal function to the Galactocentric longitude. In this research, we aim to find evidence of the transient Milky Way warp using kinematics data of the Red Giant Branch (RGB) stars from Gaia Data Release 2. We only use RGB stars with positive parallaxes and small parallax uncertainties where ϖ > 0 and ϖ/σϖ > 5, respectively. Using the linear warp model, we find the Milky Way warp starts on Rw = 10.72 kpc, with the amplitude (γw ) of 0.39 and the Line of Node lies on ϕw = −5.26°. Assumed the warp is a static long-lived feature on kinematics distribution, where all of the warp parameters are constant by the time, the observed vertical velocity (Vz ) is less than the expected one. The maximum Vz is not exceeded to 12.5 km/s rather 20 km/s derived from the proposed model. It infers that the warp drive on the Milky Way is a transient feature. One of the possible reasons is the recent interaction with satellite galaxies in perturbing the outer disk of the Milky Way.