2018ApJ...865L..19T

The perturbation mechanism of the Galactic disk is a long-standing puzzle. The imprints from perturbations provide important diagnostics concerning the disk's formation and evolution. Here we try to constrain when the vertical perturbation took place in the disk by tracking the phase-mixing history. First, we clearly depict the spiral structures of radial (v_R) and azimuthal (v_ph) velocities in the phase space of the vertical position and velocity (z-v_z) with 723,871 Large Sky Area Multi-Object Fiber Spectroscopic Telescope/Gaia combined stars. Then, we investigate the variation of the spirals with stellar age (τ) by dividing the sample into seven stellar age bins. Finally, we find that the spirals explicitly exist in all of the bins, even in the bin of τ < 0.5 Gyr, except for the bin of τ > 6.0 Gyr. This constrains the vertical perturbation to starting probably no later than 0.5 Gyr ago. But we cannot rule out the possibility that the young stars (τ < 0.5 Gyr) inherit the oscillations from the perturbed interstellar medium from where they were born. This study provides some important observational evidences to understand the disk perturbation mechanisms, and even the formation and evolution of our Galaxy.