The GALAH survey and Gaia DR2: forced oscillation and phase mixing in the local stellar disc

We use data from the second data releases of the ESA Gaia astrometric survey and the high-resolution GALAH spectroscopic survey to analyse the structure of our Galaxy's discs. With GALAH, we can separate the \alpha-rich and \alpha-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We examine carefully the distribution of stars in the z-V_z phase plane in which a remarkable ``phase spiral'' was recently discovered. We show that the spiral extends well beyond the narrow cylinder in which it was found and is present in both discs but appears most clearly in stars on less eccentric orbits. We display the phase spiral in V_\phi in addition to V_R and identify the signature in the z-V_z plane of a tilting velocity ellipsoid. The spiral is presumably a signature of a non-equilibrium stellar distribution phase mixing, but our results rule out the idea that it is associated with disrupting star clusters. We present new simulations of tidal disturbance of the Galactic disc by a halo substructure similar to the Sgr dwarf. The effects on the z-V_z plane of the substructure passing through the disc last at least 2 Gyr, but a subsequent disc crossing wipes out the coherent structure. These simulations suggest that the phase spiral was created 0.5 Gyr ago as an object like Sgr with total mass 3 x 10^10 Msun (stripped down from 5 x 10^10 Msun when it first entered the halo) passed through the plane.

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There's a whole lot of shaking going on... Elvis.