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

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

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    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≲RGC≲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.

    Original languageEnglish
    Pages (from-to)5216-5232
    Number of pages17
    JournalMonthly Notices of the Royal Astronomical Society
    Volume476
    Issue number4
    DOIs
    Publication statusPublished - 1 Jun 2018

    Bibliographical note

    This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 476, Issue 4, 1 June 2018, Pages 5216–5232, https://doi.org/10.1093/mnras/sty525. Copyright 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

    Keywords

    • surveys
    • stars: abundances
    • Galaxy: disc
    • Galaxy: evolution
    • Galaxy: formation

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