3 years ago

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

P. R. Kafle, E. Hyde, L. Casagrande, G. S. Da Costa, S. Buder, S. Sharma, T. Zwitter, D. M. Nataf, B. Anguiano, D. B. Zucker, J. Bland-Hawthorn, G. F. Lewis, V. D'Orazi, R. F. G. Wyse, W. Reid, J. Horner, M. Ness, Y.-S. Ting, K. Schlesinger, K. Lind, M. Asplund, D. Stello, K. C. Freeman, J. Lin, S. L. Martell, L. Duong, J. D. Simpson, J. Kos, G. M. De Silva

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' 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 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.

Publisher URL: http://arxiv.org/abs/1801.01514

DOI: arXiv:1801.01514v2

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