The GALAH survey: properties of the Galactic disk(s) in the solar
  neighbourhood
  Using data from the GALAH pilot survey, we determine properties of the
Galactic thin and thick disks near the solar neighbourhood. The data cover a
small range of Galactocentric radius ($7.9 \leq R_\mathrm{GC} \leq 9.5$ kpc),
but extend up to 4 kpc in height from the Galactic plane, and several kpc in
the direction of Galactic anti-rotation (at longitude $260 ^\circ \leq \ell
\leq 280^\circ$). This allows us to reliably measure the vertical density and
abundance profiles of the chemically and kinematically defined `thick' and
`thin' disks of the Galaxy. The thin disk (low-$\alpha$ population) exhibits a
steep negative vertical metallicity gradient, at d[M/H]/d$z=-0.18 \pm 0.01$ dex
kpc$^{-1}$, which is broadly consistent with previous studies. In contrast, its
vertical $\alpha$-abundance profile is almost flat, with a gradient of
d[$\alpha$/M]/d$z$ = $0.008 \pm 0.002$ dex kpc$^{-1}$. 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 disk. The
thick disk (high-$\alpha$ population) has a weaker vertical metallicity
gradient d[M/H]/d$z = -0.058 \pm 0.003$ dex kpc$^{-1}$. The $\alpha$-abundance
of the thick disk is nearly constant with height, d[$\alpha$/M]/d$z$ = $0.007
\pm 0.002$ dex kpc$^{-1}$. The negative gradient in metallicity and the small
gradient in [$\alpha$/M] indicate that the high-$\alpha$ population experienced
a settling phase, but also formed prior to the onset of major SNIa enrichment.
We explore the implications of the distinct $\alpha$-enrichments and narrow
[$\alpha$/M] range of the sub-populations in the context of thick disk
formation.
