Snails Across Scales: characterizing phase-space spirals in N-body simulations

The discovery of phase-space spirals from projections of the z-vz plane in the Gaia data by Antoja et al. (2018) opened up the exciting possibility that detailed stellar kinematics in the local Galactic disk can be used to study global interactions between the Milky Way and its satellites. The morphology and spatial dependence of these phase-space spirals and other signatures of disequilibrium encodes information both about the shape of the Milky Way’s potential and the specifics of the perturbation and interaction. However, any interpretation must rely on the properties of the spirals themselves and robustly extracting these signatures is challenging. Here, we present an algorithm designed to characterize the strength and morphology of phase-space spirals. We test the algorithm on self-consistent simulations of the Milky Way–Sagittarius merger. We show that while the physics behind spiral formation is conceptually simple, interactions between the Milky Way and an outside perturber can cause complicated dynamical responses in the disk. However, by characterizing these spirals as a function of location, we can start to piece together the merger history of the Milky Way.