Origin of the Gaia phase spiral in the Milky Way disk
In plain words
Gaia showed that nearby stars, plotted by height above the disk against vertical speed, trace a snail-shell spiral. It may be the ringing from a passing dwarf galaxy or from the Milky Way's own bar and arms.
Precise statement
Identify the perturbation behind the $z-v_{z}$ phase spiral discovered in Gaia DR2 (Antoja et al. 2018): a Sagittarius dwarf pericenter passage, bar buckling, dark subhalos, or many stochastic perturbations, using its winding time, dependence on radius, age and guiding radius, and the satellite's mass.
What would settle it
Self-consistent N-body models of the Milky Way with a realistic Sagittarius mass history that reproduce the spiral's amplitude, winding and radial dependence.
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