Lunch talk on May. 13th, 2019
Dissecting the phase space snail
Speaker: Zhaoyu Li (SJTU/SHAO)
Venue: Room 2317, SWIFAR Building
Time: 12:30 PM, Monday, 13th May, 2019
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 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 Sun. Surprisingly, the phase space snail shell exists only in the main kinematic streams with |V_R| < 50 km/s and |V_phi - V_LSR| < 30 km/s, i.e., stars on cold in-plane orbits. No snail shell is discernible for stars on the otherwise hot in-plane orbits, which may have phase wrapped away already.
Moreover, we confirm that the Hercules stream has two branches (at fast and slow V_phi). The two branches cannot be explained by a single mechanism, since the phase space snail shell can only be seen in the fast branch, not in the slow one, indicating different orbital compositions between the two branches.
In addition, we find that the snail shell is not clear in the V_phi color-coded phase space for the streams in cold orbits. The prominent V_phi color-coded snail shell observed in previous work is simply due to the number weighted average of the azimuthal velocities of these streams, with the stars in the hot orbits providing a smooth background to highlight the snail shell of the stars in the cold orbits. Therefore, the weak V_phi color-coded snail shell indicates that the correlation between the in-plane and vertical motions might not be as strong as previously suggested.