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

Spectacular evidence of a coherent pattern in phase space has emerged in the local stellar disc from an analysis of 6 million stars in the ESA Gaia Data Release 2 (Gaia DR2). This remarkable astrometric mission is complemented by the ongoing GALAH survey of the Milky Way that exploits the HERMES high-resolution (R 28, 000), multi-object spectrograph at the Anglo-Australian Telescope. In its latest release, GALAH DR2 provides accurate radial velocities ( r v 0.1 -0.2 km s -1 ) and stellar abundances (for up to 23 elements) for 342,682 stars, all of which overlap with the Gaia catalogue. If (V R , V , V z ) are the components of velocity in Galactic cylindrical coordinates (R, , z), the phase-wrapping signature is seen most clearly in the vertical phase plane (z, V z ) when averaging over V for all Gaia stars in the local volume element as a function of the phase space coordinates, i.e. V (z, V z ) . A weaker spiral phase is also seen in the V R (z, V z ) plane superimposed on a quadrupole pattern due to the tilt of the local velocity ellipsoid. The one-arm (m = 1) spiral trajectory in phase space (which we abbreviate to 'spiral phase') is recovered by the GALAH (and LAMOST) survey in both the thin disc and thick disc population defined in the [Fe/H] vs. [/Fe] plane. The stellar abundances moving out from the centre of the 3D spiral phase reflect the vertical and radial metallicity gradients. We conduct simulations of the Milky Way being hit by the Sgr dwarf using a range of impactor masses (1 -10 10 10 M ), all of which lose mass and cross the disc multiple times. In regions where the disc-crossing timescale ( C ) is comparable to the disc's vertical period ( C 2/ z ), Sgr drives a forced oscillation in the disc population resulting in 'bending waves' across the disc. This produces a rotating spiral pattern in the (z, V z ) plane seen in both V R and V that does not wind up on large scales. At smaller radii and close to the disc plane for which C 2/ z , phase-wrapping and mixing does occur. The amplitude and contrast of the phase pattern observed today is consistent with an impactor mass of at least 3 10 10 M at transit stripped down from 5 10 10 M when it crossed the virial radius. Each successive disc transit washes out the spiral phase signal but it reforms in about 100 -150 Myr before the next crossing. Since the last crossing occurred about 400 Myr ago, the spiral phase cannot be older than about 250 -300 Myr.