First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33

Roeland P. Van Der Marel, Mark A. Fardal, Sangmo Tony Sohn, Ekta Patel, Gurtina Besla, Andrés Del Pino, Johannes Sahlmann, Laura L. Watkins

Research output: Contribution to journalArticlepeer-review

78 Scopus citations


The 3D velocities of M31 and M33 are important for understanding the evolution and cosmological context of the Local Group. Their most massive stars are detected by Gaia, and we use Data Release 2 (DR2) to determine the galaxy proper motions (PMs). We select galaxy members based on, e.g., parallax, PM, color-magnitude diagram location, and local stellar density. The PM rotation of both galaxies is confidently detected, consistent with the known line-of-sight rotation curves: (counterclockwise) for M31, and (clockwise) for M33. We measure the center-of-mass PM of each galaxy relative to surrounding background quasars in DR2. This yields that equals for M31 and for M33. In addition to the listed random errors, each component has an additional residual systematic error of . These results are consistent at 0.8σ and 1.0σ with the (2 and 3 times higher accuracy) measurements already available from Hubble Space Telescope (HST) optical imaging and Very Long Baseline Array water maser observations, respectively. This lends confidence that all these measurements are robust. The new results imply that the M31 orbit toward the Milky Way (MW) is somewhat less radial than previously inferred, , and strengthen arguments that M33 may be on its first infall into M31. The results highlight the future potential of Gaia for PM studies beyond the MW satellite system.

Original languageEnglish (US)
Article number24
JournalAstrophysical Journal
Issue number1
StatePublished - Feb 10 2019


  • Local Group
  • galaxies: kinematics and dynamics
  • proper motions

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'First Gaia Dynamics of the Andromeda System: DR2 Proper Motions, Orbits, and Rotation of M31 and M33'. Together they form a unique fingerprint.

Cite this