The GALAH survey and Gaia DR2: Dissecting the stellar disc's phase space by age, action, chemistry, and location

Joss Bland-Hawthorn, Sanjib Sharma, Thor Tepper-Garcia, James Binney, Ken C. Freeman, Michael R. Hayden, Janez Kos, Gayandhi M. De Silva, Simon Ellis, Geraint F. Lewis, Martin Asplund, Sven Buder, Andrew R. Casey, Valentina D'Orazi, Ly Duong, Shourya Khanna, Jane Lin, Karin Lind, Sarah L. Martell, Melissa K. NessJeffrey D. Simpson, Daniel B. Zucker, Tomaž Zwitter, Prajwal R. Kafle, Alice C. Quillen, Yuan Sen Ting, Rosemary F.G. Wyse, the GALAH team

Research output: Contribution to journalReview ArticleResearchpeer-review

27 Citations (Scopus)


We use the second data releases of the European Space AgencyGaia astrometric survey and the high-resolution Galactic Archaeology with HERMES (GALAH) spectroscopic survey to analyse the structure of our Galaxy's disc components. With GALAH, we separate the α-rich and α-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We study the distribution of stars in the zVz phase plane, for both Vφ and VR, and recover the remarkable 'phase spiral' discovered by Gaia. We identify the anticipated quadrupole signature in zVz of a tilted velocity ellipsoid for stars above and below the Galactic plane. By connecting our work with earlier studies, we show that the phase spiral is likely to extend well beyond the narrow solar neighbourhood cylinder in which it was found. The phase spiral is a signature of corrugated waves that propagate through the disc, and the associated non-equilibrium phase mixing. The radially asymmetric distribution of stars involved in the phase spiral reveals that the corrugation, which is mostly confined to the α-poor disc, grows in z-amplitude with increasing radius. We present new simulations of tidal disturbance of the Galactic disc by the Sagittarius (Sgr) dwarf. The effect on the zVz phase plane lasts ≥2Gyr, but a subsequent disc crossing wipes out the coherent structure. We find that the phase spiral was excited ≤0.5 Gyr ago by an object like Sgr with total mass ∼3 × 1010 M (stripped down from ∼5 × 1010 M when it first entered the halo) passing through the plane.

Original languageEnglish
Pages (from-to)1167-1191
Number of pages25
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - Jun 2019


  • astrometry
  • Galaxy: disc, evolution, structure
  • proper motions
  • stars: kinematics and dynamics

Cite this

Bland-Hawthorn, J., Sharma, S., Tepper-Garcia, T., Binney, J., Freeman, K. C., Hayden, M. R., Kos, J., De Silva, G. M., Ellis, S., Lewis, G. F., Asplund, M., Buder, S., Casey, A. R., D'Orazi, V., Duong, L., Khanna, S., Lin, J., Lind, K., Martell, S. L., ... the GALAH team (2019). The GALAH survey and Gaia DR2: Dissecting the stellar disc's phase space by age, action, chemistry, and location. Monthly Notices of the Royal Astronomical Society, 486(1), 1167-1191.