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

Duong, L. and Freeman, K. C. and Asplund, M. and Casagrande, L. and Buder, S. and Lind, K. and Ness, M. and Bland-Hawthorn, J. and de Silva, G. M. and D'Orazi, V. and Kos, J. and Lewis, G. F. and Lin, J. and Martell, S. L. and Schlesinger, K. and Sharma, S. and Simpson, J. D. and Zucker, D. B. and Zwitter, T. and Anguiano, B. and Da Costa, G. S. and Hyde, E. and Horner, J. and Kafle, P. R. and Nataf, D. M. and Reid, W. and Stello, D. and Ting, Y.-S. and Wyse, R. F. G. (2018) The GALAH survey: properties of the Galactic disc(s) in the solar neighbourhood. Monthly Notices of the Royal Astronomical Society, 476 (4). pp. 5216-5232. ISSN 0035-8711

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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<SUP>-1</SUP>, 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<SUP>-1</SUP>. 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<SUP>-1</SUP>. The α-abundance of the thick disc is nearly constant with height, d[α/M]/dz = 0.007 ± 0.002 dex kpc<SUP>-1</SUP>. 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.

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Item Type: Article (Commonwealth Reporting Category C)
Refereed: Yes
Item Status: Live Archive
Additional Information: Published version deposited in accordance with the copyright policy of the publisher.
Faculty / Department / School: Current - Faculty of Health, Engineering and Sciences - School of Agricultural, Computational and Environmental Sciences
Date Deposited: 15 Feb 2019 02:26
Last Modified: 04 Mar 2019 04:09
Uncontrolled Keywords: surveys; stars: abundances; Galaxy: disc; Galaxy: evolution; Galaxy:; formation; Astrophysics - Astrophysics of Galaxies
Fields of Research : 02 Physical Sciences > 0201 Astronomical and Space Sciences > 020110 Stellar Astronomy and Planetary Systems
Identification Number or DOI: 10.1093/mnras/sty525

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