Determine the origin of the Milky Way’s phase-spirals

Determine the physical origin of the Milky Way’s observed vertical phase-spirals, including whether they were generated by the Sagittarius dwarf galaxy, Galactic bar buckling, spiral arms, a dynamical-friction wake, substructure, or misaligned gas accretion.

Background

The paper explains that vertical phase-spirals encode information about perturbations to the Galactic disk, including the perturbation’s symmetry, strength, timing, and location. Several mechanisms have been proposed, most prominently passages of the Sagittarius dwarf galaxy through the disk, but the authors state that the cause of the observed Milky Way phase-spirals has not been established. Resolving this origin problem is important for reconstructing the Galaxy’s recent merger history and dynamical evolution.

References

Despite this seemingly straightforward mapping from a phase-spiral to its origin, the cause of the Milky Way's phase-spirals remains unknown.

— Phase-Spirals Across Galactic Disks II: Using large-scale "macro-spirals" in phase-spiral amplitude to derive perturbation times  (2609.09543 - Tavangar et al., 8 Sep 2026) in Section 1, Introduction

Note that we cannot verify that the peaks at a single azimuth are indeed contiguously connected around the disk to form an $m=1$ spiral.

— Phase-Spirals Across Galactic Disks II: Using large-scale "macro-spirals" in phase-spiral amplitude to derive perturbation times  (2609.09543 - Tavangar et al., 8 Sep 2026) in Section 3.2, “Milky Way macro-spiral results”

There is currently no consensus about the observed phase-spirals' origins, including i) what event (or events) seeded these structures and ii) when did this event (or these events) happen.

— Phase-Spirals Across Galactic Disks II: Using large-scale "macro-spirals" in phase-spiral amplitude to derive perturbation times  (2609.09543 - Tavangar et al., 8 Sep 2026) in Section 5.2, “Future prospects: in theory”