Vertical Phase Mixing across the Galactic Disk

Abstract: By combining the {\it LAMOST} and {\it Gaia} data, we investigate the vertical phase mixing across the Galactic disk. Our results confirm the existence of the phase space snail shells (or phase spirals) from 6 to 12 kpc. We find that grouping stars by the guiding radius ($R_{g}$), instead of the present radius ($R$) further enhances the snail shell signal in the following aspects: (1) clarity of the snail shell shape is increased; (2) more wraps of the snail shell can be seen; (3) the phase spaces are less affected by the lack of stars closer to the disk mid-plane due to extinction; (4) the phase space snail shell is amplified in greater radial ranges. Compared to the $R$-based snail shell, the quantitatively measured shapes are similar, except that the $R_{g}$-based snail shells show more wraps with better contrast. These lines of evidence lead to the conclusion that the guiding radius (angular momentum) is a fundamental parameter tracing the phase space snail shell across the Galactic disk. Results of our test particle simulations with impulse approximation verify that particles grouped according to $R_{g}$ reveal well-defined and sharper snail shell features. By comparing the radial profiles of the pitch angle between observation and simulation, the external perturbation can be constrained to $\sim$500$-$700 Myr ago. For future vertical phase mixing study, it is recommended to use the guiding radius with additional constraints on orbital hotness (ellipticity) to improve the clarity of the phase snail.