Phase-Spirals Across Galactic Disks II: Using large-scale "macro-spirals" in phase-spiral amplitude to derive perturbation times

Abstract: Phase-space spirals in the Milky Way disk are a key observable remnant of recent perturbations to the Galaxy. They provide insight into the disk's potential, dynamical evolution, past interactions, and even substructure. However, the complex dynamics of phase-spiral formation and evolution have made clear interpretations challenging. For example, recent work has shown that the "winding time" -- measured from how tightly wound a phase-spiral is -- is a biased estimate of the true time since the inciting perturbation due to the complex effects of self-gravity. In this paper series, we present an alternative approach by looking at correlations in phase-spiral morphology across the Galactic disk. Here we show that following a localized perturbative event, the ridgeline connecting the largest amplitude phase-spiral at each radius winds up into a "macro-spiral" at the rate expected for differential rotation. This means the macro-spiral can 1) be unwound to constrain the origin -- time and location -- of correlated phase-spiral properties and 2) indicate the delay in individual phase-spiral winding as a novel diagnostic of disk dynamical properties. Applying these ideas to the Milky Way's phase-spirals, we estimate a perturbation time of $\simeq 1$ Gyr ago, consistent with the penultimate passage of the Sagittarius dwarf galaxy through the disk, and delay times of up to 800 Myr for phase-spirals in the inner disk. While the current application of this method to Gaia DR3 data is limited by the available radial velocities, future Gaia data releases will enable stronger constraints using a much larger area of the Galactic disk.