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Thomas Bensby. Profile photo.

Thomas Bensby

Senior lecturer

Thomas Bensby. Profile photo.

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


  • S. Alinder
  • P. J. McMillan
  • T. Bensby

Summary, in English

Context. With data releases from the astrometric space mission Gaia, exploration of the structure of the Milky Way is now possible in unprecedented detail, and has unveiled many previously unknown structures in the Galactic disc and halo. One such feature is the Gaia phase spiral where the stars in the Galactic disc form a spiral density pattern in the Z- VZ plane. Many questions regarding the phase spiral remain, particularly how its amplitude and rotation change with position in the Galaxy. Aims. We aim to characterize the shape, rotation, amplitude, and metallicity of the phase spiral in the outer disc of the Milky Way. This will allow us to better understand which physical processes caused the phase spiral and may provide further clues as to the Milky Way s past and the events that contributed to its current state. Methods. We use Gaia data release 3 (DR3) to get full position and velocity data on approximately 31.5 million stars, and metallicity for a subset of them. We then compute the angular momenta of the stars and develop a model to characterise the phase spiral in terms of amplitude and rotation at different locations in the disc. Results. We find that the rotation angle of the phase spiral changes with Galactic azimuth and galactocentric radius, making the phase spiral appear to rotate about 3 per degree in Galactic azimuth. Furthermore, we find that the phase spiral in the 2200- 2400 kpc km s- 1 range of angular momentum is particularly strong compared to the phase spiral that can be observed in the solar neighbourhood. The metallicity of the phase spiral appears to match that of the field stars of the Milky Way disc. Conclusions. We created a new model capable of fitting several key parameters of the Gaia phase spiral. We have been able to determine the rotation rate of the phase spiral to be about 3 per degree in Galactic azimuth. We find a maximum in the amplitude of the phase spiral at LZ 2300 km kpc s- 1, which makes the phase spiral clearly visible. These results provide insights into the physical processes that led to the formation of the phase spiral and contribute to our understanding of the Milky Way s past and present state.


  • Astrophysics

Publishing year





Astronomy and Astrophysics



Document type

Journal article


EDP Sciences


  • Astronomy, Astrophysics and Cosmology


  • Galaxy: disk
  • Galaxy: evolution
  • Galaxy: kinematics and dynamics
  • Galaxy: structure
  • Solar neighborhood




  • ISSN: 0004-6361