The GALAH survey: properties of the Galactic disk(s) in the solar neighbourhood


Abstract in English

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.

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