A 10 000 star spectroscopic survey of the thick disc-halo interface: Phase-space sub-structure in the thick disc

We analyse a 10 000 star spectroscopic survey, focused on Galactic thick disc stars typically 2-5 kpc from the Sun, carried out using the AAOmega Spectrograph on the Anglo-Australian Telescope (AAT). We develop methods for completeness correction of the survey based on Sloan Digital Sky Survey (SDSS) photometry, and we derive star distances using an improved isochrone fitting method with accuracies better than 10 per cent. We determine the large-scale kinematic [Vφ = 172 km s^-1 and σ(φ, r, z) = (49,51,40) km s^-1], and abundance properties of the thick disc, showing these representative values are a fair description within about 3 kpc of the Sun, and in the range 1-3 kpc from the Galactic plane. We identify a substantial overdensity in lines of sight towards the inner Galaxy, with metallicity [Fe/H] ~ -1 dex, and higher line of sight velocities than the thick disc, localized along the direction (l, b) = (48{ring operator}, -26{ring operator}). This overdensity appears to be towards, but closer than, the known Hercules-Aquila halo overdensity and may be related to the Humphreys-Larsen inner Galaxy thick disc asymmetry.