The Phase Spiral’s Origin and Evolution: Indications from Its Varying Properties across the Milky Way Disk

Abstract The phase spiral is a perturbation to the vertical phase-space distribution of stars in the Milky Way disk. We study the phase spiral’s properties and how they vary with spatial position, in order to constrain its origin and evolution, as well as properties of the disk itself. We produce high-resolution maps using two complementary data processing schemes: (a) we bin the Gaia proper-motion sample in a disk-parallel spatial grid, reaching distances up to 4 kpc; and (b) we bin the spatially nearby line-of-sight velocity sample in terms of disk-parallel orbital parameters. We find complex structure, most significantly with respect to Galactocentric radius and guiding radius, but also in Galactic azimuth and epicyclic action and phase. We find that spiral winding and rotation phase vary smoothly across the disk, with close-to-flat radial profiles. This uniform structure, in particular for the rotation phase, disfavors small-scale perturbations as the primary source of the phase spiral, for example, from giant molecular clouds or dark matter subhalos. Curiously, these close-to-flat profiles also imply that the winding time has a strong slope with respect to Galactocentric radius, with low values for the inner disk. A uniform perturbation time cannot be reconciled by self-gravity effects that act only in the disk’s vertical dimension, because this would delay winding but not the evolution of the rotation phase. This suggests that more complex self-gravity effects, acting also in dimensions parallel to the disk plane, are important for the spiral’s formation and evolution.