The GALAH survey: properties of the Galactic disc(s) in the solar neighbourhood
Date Issued
2018
Author(s)
Duong, L.
•
Freeman, K. C.
•
Asplund, M.
•
Casagrande, L.
•
Buder, S.
•
Lind, K.
•
Ness, M.
•
Bland-Hawthorn, J.
•
De Silva, G. M.
•
•
Kos, J.
•
Lewis, G. F.
•
Lin, J.
•
Martell, S. L.
•
Schlesinger, K.
•
Sharma, S.
•
Simpson, J. D.
•
Zucker, D. B.
•
Zwitter, T.
•
Anguiano, B.
•
Da Costa, G. S.
•
Hyde, E.
•
Horner, J.
•
Kafle, P. R.
•
Nataf, D. M.
•
Reid, W.
•
Stello, D.
•
Ting, Y. -S.
•
Wyse, R. F. G.
Abstract
Using data from the GALAH pilot survey, we determine properties of the Galactic thin and thick discs near the solar neighbourhood. The data cover a small range of Galactocentric radius (7.9 ≲ R_GC ≲ 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° ≤ ℓ ≤ 280°). 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-α population) exhibits a steep negative vertical metallicity gradient, at d[M/H]/dz = -0.18 ± 0.01 dex kpc-1, which is broadly consistent with previous studies. In contrast, its vertical α-abundance profile is almost flat, with a gradient of d[α/M]/dz = 0.008 ± 0.002 dex kpc-1. The steep vertical metallicity gradient of the low-α population is in agreement with models where radial migration has a major role in the evolution of the thin disc. The thick disc (high-α population) has a weaker vertical metallicity gradient d[M/H]/dz = -0.058 ± 0.003 dex kpc-1. The α-abundance of the thick disc is nearly constant with height, d[α/M]/dz = 0.007 ± 0.002 dex kpc-1. The negative gradient in metallicity and the small gradient in [α/M] indicate that the high-α population experienced a settling phase, but also formed prior to the onset of major Type Ia supernova enrichment. We explore the implications of the distinct α-enrichments and narrow [α/M] range of the sub-populations in the context of thick disc formation.
Volume
476
Issue
4
Start page
5216
Issn Identifier
0035-8711
Ads BibCode
2018MNRAS.476.5216D
Rights
open.access
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