http://arxiv.org/abs/1801.01514
Using data from the GALAH pilot survey, we determined properties of the Galactic thin and thick disk near the solar neighbourhood. The data cover a small range of Galactocentric radius ($7.9 \leq R_\mathrm{GC} \leq 9.5$ kpc), but extends up to 4 kpc in height from the Galactic plane, and several kpc in the direction of Galactic anti-rotation. This allows us to measure reliably the vertical density and abundance profiles of the chemically and kinematically defined
thick' andthin’ disks of the Galaxy. We found scale heights of $0.40 \pm 0.02$ kpc and $0.81 \pm 0.04$ kpc for the low- and high-$\alpha$ populations, respectively. 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 metallicity gradient of the low-$\alpha$ population seems consistent with various radial migration predictions and further confirms that migration plays an important 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 SNeIa 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.
L. Duong, K. Freeman, M. Asplund, et. al.
Mon, 8 Jan 18
98/117
Comments: 16 pages, 15 figures