Dissecting the Phase Space Snail Shell. (arXiv:1904.03314v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Li_Z/0/1/0/all/0/1">Zhao-Yu Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shen_J/0/1/0/all/0/1">Juntai Shen</a> (Shanghai Jiao Tong Univ. & Shanghai Astronomical Obs.)
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.
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.
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