Thick disc probability of GALAH stars

The data cover a small range of Galactocentric radius (7.9<~R_GC_<~9.5kpc), but extend up to 4kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation (at longitude 260{deg}<=l<=280{deg}). This allows us to reliably measure the vertical density and abundance profiles of the chemically and kinematically defined 'thick' and 'thin' discs of the Galaxy.

The thin disc (low-{alpha} population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/dz=-0.18+/-0.01dex/kpc, which is broadly consistent with previous studies. In contrast, its vertical {alpha}-abundance profile is almost flat, with a gradient of d[{alpha}/M]/dz=0.008+/-0.002dex/kpc. The steep vertical metallicity gradient of the low-{alpha} population is in agreement with models where radial migration has a major role in the evolution of the thin disc.