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Title: Origin of the Gaia phase spiral in the Milky Way disk
Plain: 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: 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.
Settled by: 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.
Kind: well-posed
Answer type: identification
References: (empty)
Literature status: contested
Status note: (empty)
Posed since: 2018