# Dissecting the Phase Space Snail Shell

**Authors:** Zhao-Yu Li, Juntai Shen  
**arXiv:** [1904.03314](https://arxiv.org/abs/1904.03314)  
**Date:** 2019-04-05  
**Summary page:** https://www.summarizepaper.com/en/paper/1904.03314v1/

## Abstract

The on-going phase mixing in the vertical direction of the Galactic disk has been discovered with the revolutionary Gaia DR2 data. It manifests itself as the snail shell in the $Z-V_{Z}$ phase space. To better understand the origin and properties of the phase mixing process, we study the phase-mixing signatures in moving groups (also known as the kinematic streams) with the Gaia DR2 data in the Galactic disk near the Solar circle. Interestingly, the phase space snail shell exists only in the main kinematic streams with $|V_{R}| \lesssim 50$ km/s and $|V_{\phi} - V_{\rm LSR}| \lesssim 30$ km/s, i.e., stars on dynamically ``colder'' orbits. No snail shell is discernible for stars on hotter orbits. Thus one should pay closer attention to the colder orbits in future phase mixing studies. We also confirm that the Hercules stream has two branches (at fast and slow $V_{\phi}$), which may not be explained by a single mechanism, since the phase space snail shell is only seen in the fast branch, but not in the slow one. Compared to the colder orbits, the hotter orbits may have phase-wrapped away already due to the much larger dynamical range in radial variation to facilitate faster phase mixing. These results help put tighter constraints on the vertical perturbation history of the Milky Way disk. To explain the lack of a well-defined snail shell in the hotter orbits, the disk should be perturbed at least $\sim 400-500$ Myr ago. Our results offer more support to the external perturbation scenario than the internal bar buckling perturbation scenario as the origin of the phase space mixing. The more prominent snail shell in the $V_{\phi}$ color-coded phase space is due partly to the fact that only the colder orbits exhibit the phase space spiral, with the featureless distribution of the hotter orbits providing a contrasting background.