# Investigating the amplitude and rotation of the phase spiral in the Milky Way outer disc

**Authors:** S. Alinder, P. J. McMillan, T. Bensby  
**arXiv:** [2303.18040](https://arxiv.org/abs/2303.18040)  
**Date:** 2023-03-31  
**Summary page:** https://www.summarizepaper.com/en/paper/2303.18040v1/

## Key points

- Data releases from the Gaia mission have enhanced the exploration of the Milky Way's structure.
- The phase spiral is a previously unknown structure in the Galactic disc and halo.
- The authors aim to characterize various properties of the phase spiral in the outer disc.
- They utilize data from Gaia data release 3 (DR3) for their analysis.
- The rotation angle of the phase spiral changes with Galactic azimuth and Galactocentric radius.
- A particularly strong phase spiral is observed within a specific range of angular momentum.
- The metallicity of this phase spiral matches that of Milky Way disc field stars.
- The study presents a new model capable of fitting key parameters related to the phase spiral.
- The authors determine its rotation rate and identify a peak in its amplitude.

## Summary

The exploration of the structure of the Milky Way has been greatly enhanced by the data releases from the astrometric space mission Gaia. These releases have provided unprecedented detail and revealed previously unknown structures in both the Galactic disc and halo. One such structure is the phase spiral, where stars in the Galactic disc form a spiral density pattern in the $Z-V_Z$ plane. In this study, the authors aim to characterize various properties of the phase spiral in the outer disc of the Milky Way. By understanding these properties, they hope to gain insights into the physical processes that led to its formation and shed light on the past events that have shaped our galaxy's current state. To achieve their goals,  utilize data from Gaia data release 3 (DR3), which includes full position and velocity information for approximately 31.5 million stars, as well as metallicity data for a subset of them. They compute angular momenta for these stars and develop a model to characterize  phase spiral in terms of its amplitude and rotation at different locations within  disc. The results of their analysis reveal interesting findings about  phase spiral. They observe that its rotation angle changes with Galactic azimuth and Galactocentric radius, causing it to appear to rotate about $3^\circ$ per degree in Galactic azimuth. Additionally, they find that there is a particularly strong phase spiral within a specific range of angular momentum (2200-2400 kpc km s$^{-1}$) compared to what can be observed in our solar neighborhood. The metallicity of this phase spiral matches that of Milky Way disc field stars. In conclusion, this study presents a new model capable of fitting several key parameters related to  phase spiral in our galaxy's outer disc. The authors successfully determine its rotation rate and identify a peak in its amplitude, which manifests as a clear phase spiral when considering only stars with similar angular momentum. These findings contribute to our understanding of the formation and evolution of large spiral galaxies, providing valuable insights into the processes that have shaped the Milky Way.