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

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 find clear evidence that young ( 0.1) dwarfs have migrated from the inner alpha-poor disc to the solar neighbourhood. We examine 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 quadrupole signature in the z-V_z plane of a tilting velocity ellipsoid. The spiral is a signature of corrugated waves that propagate through the disc, and the associated non-equilibrium phase mixing; our results rule out the idea that it is associated with disrupting star clusters. The radially asymmetric distribution of stars involved in the phase spiral shows that the corrugation, which is mostly confined to the alpha-poor disc, grows in z-amplitude as a function of increasing radius. We present new simulations of tidal disturbance of the Galactic disc by a halo substructure similar to the Sagittarius (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 500 Myr ago as an object like Sgr with total mass 3x10^{10} Msun (stripped down from 5x10^{10} Msun when it first entered the halo) passed through the plane.