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Peer-reviewedCC BY 4.0The Astrophysical Journal

Phase Spirals across Galactic Disks. II. Using Large-scale “Macrospirals” in Phase-spiral Amplitude to Derive Perturbation Times

  • Kiyan Tavangar1
  • Kathryn V. Johnston1
  • Jason A. S. Hunt2
  • Axel Widmark1,3
  • Vandana G. Kaushik2
  • Adrian M. Price-Whelan4
4 affiliations
  1. 1Columbia University
  2. 2University of Surrey
  3. 3Stockholm University and The Oskar Klein Centre for Cosmoparticle Physics
  4. 4Flatiron Institute
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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 galactic disks. Here, we show that following a localized perturbative event, the ridge line connecting the largest amplitude phase spiral at each radius winds up into a “macrospiral” at the rate expected for differential rotation. This means the macrospiral 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 ≃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.

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Cite

BibTeXRIS
BibTeX
@article{tavangar2026phase,
  title = {{Phase Spirals across Galactic Disks. II. Using Large-scale “Macrospirals” in Phase-spiral Amplitude to Derive Perturbation Times}},
  author = {Kiyan Tavangar and Kathryn V. Johnston and Jason A. S. Hunt and Axel Widmark and Vandana G. Kaushik and Adrian M. Price-Whelan},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea6b1},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea6b1},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Phase Spirals across Galactic Disks. II. Using Large-scale “Macrospirals” in Phase-spiral Amplitude to Derive Perturbation Times
AU  - Kiyan Tavangar
AU  - Kathryn V. Johnston
AU  - Jason A. S. Hunt
AU  - Axel Widmark
AU  - Vandana G. Kaushik
AU  - Adrian M. Price-Whelan
PY  - 2026
DA  - 2026/10/12
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea6b1
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea6b1
AB  - 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 galactic disks. Here, we show that following a localized perturbative event, the ridge line connecting the largest amplitude phase spiral at each radius winds up into a “macrospiral” at the rate expected for differential rotation. This means the macrospiral 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 ≃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.
SN  - 0004-637X
SN  - 1538-4357
ER  -