Snails Across Scales: Local and Global Phase-Mixing Structures as Probes of the Past and Future Milky Way

Abstract

Signatures of vertical disequilibrium have been observed across the Milky Way's disk. These signatures manifest locally as unmixed phase-spirals in z-v z space ("snails-in-phase") and globally as nonzero mean z and v z which wraps around as a physical spiral across the x-y-plane ("snails-inspace"). We explore the connection between these local and global spirals through the example of a satellite perturbing a test-particle Milky Way (MW)-like disk. We anticipate our results to broadly apply to any vertical perturbation.Using a z-v z -asymmetry metric we demonstrate that in test-particle simulations: (a) multiple local phase-spiral morphologies appear when stars are binned by azimuthal action J φ , excited by a single event (in our case, a satellite disk-crossing); (b) these distinct phase-spirals are traced back to distinct disk locations; and (c) they are excited at distinct times. Thus, local phase-spirals offer a global view of the MW's perturbation history from multiple perspectives.Using a toy model for a Sagittarius (Sgr)-like satellite crossing the disk, we show that the full interaction takes place on timescales comparable to orbital periods of disk stars within R 10 kpc. Hence such perturbations have widespread influence which peaks in distinct regions of the disk at different times.This leads us to examine the ongoing MW-Sgr interaction. While Sgr has not yet crossed the disk (currently, z Sgr ≈ −6 kpc, v z,Sgr ≈ 210 kms −1 ), we demonstrate that the peak of the impact has already passed. Sgr's pull over the past 150 Myr creates a global v z signature with amplitude ∝ M Sgr , which might be detectable in future spectroscopic surveys.

Resolving local and global kinematic signatures of satellite mergers with billion particle simulations

“…As shown in Li (2021) and Gandhi et al (2021), the local phase snail as selected in spatial coordinates is a composite of phase snails with different angular momentum, J φ . The top panel of Figure 5 shows the local phase spiral as defined as all stars within 1 kpc from the 'Solar position' defined as (x, y, z) = (−8.178, 0, 0) in model M1, where the vz and z axes have been re-scaled by the vertical velocity dispersion σv z and the vertical dispersion σz of the sample, respectively.…”

“…The middle row shows the split of stars from this sample into different angular momentum bins. The morphology of the lower row evolves with J φ as shown in Li (2021) and Gandhi et al (2021), because the phase spiral for stars selected purely in position space is an amalgamation of phase spirals from stars with different actions and different orbital histories which were located in different regions of the galaxy during the satellite perturbation (see Gandhi et al 2021, for an illustration).…”

“…A lot of attention has been paid to the local z − vz spiral since its discovery by Antoja et al (2018), but there also exists a spiral pattern in vertical position and motion which arises across the disc plane (e.g. Gómez et al 2013;Laporte et al 2019;Gandhi et al 2021). It is difficult to resolve this global kinematic spiral in the actual Milky Way data, as we struggle to track the kinematics far from the Solar neighborhood (although see Eilers et al 2020), but there is no reason we can not examine it further in the simulations, and it is of course merely a different projection of the same vertical motion, and the same vertical asymmetry.…”

Resolving local and global kinematic signatures of satellite mergers with billion particle simulations

“…The middle row shows the split of stars from this sample into different angular momentum bins. The morphology of the lower row evolves with J φ as shown in Li (2021) and Gandhi et al (2021), because the phase spiral for stars selected purely in position space is an amalgamation of phase spirals from stars with different actions and different orbital histories which were located in different regions of the galaxy during the satellite perturbation (see Gandhi et al 2021, for an illustration).…”

“…As shown in Li (2021) and Gandhi et al (2021), the local phase snail as selected in spatial coordinates is a composite of phase snails with different angular momentum, J φ . The top panel of Figure 5 shows the local phase spiral as defined as all stars within 1 kpc from the 'Solar position' defined as (x, y, z) = (−8.178, 0, 0) in model M1, where the vz and z axes have been re-scaled by the vertical velocity dispersion σv z and the vertical dispersion σz of the sample, respectively.…”