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Age-chemical abundance structure of the Galaxy I: evidence for a late accretion event in the outer disc at z ~ 0.6

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We investigate the age–chemical abundance structure of the outer Galactic disc at a galactocentric distance of r > 10 kpc as recently revealed by the SDSS/APOGEE survey. Two sequences are present in the [α/Fe]–[Fe/H] plane with systematically different stellar ages. Surprisingly, the young sequence is less metal rich, suggesting a recent dilution process by additional gas accretion. As the stars with the lowest iron abundance in the younger sequence also show an enhancement in α-element abundance, the gas accretion event must have involved a burst of star formation. In order to explain these observations, we construct a chemical evolution model. In this model, we include a relatively short episode of gas accretion at late times on top of an underlying secular accretion over long time-scales. Our model is successful at reproducing the observed distribution of stars in the three-dimensional space of [α/Fe]–[Fe/H]–age in the outer disc. We find that a late-time accretion with a delay of 8.28.2Gyr and a time-scale of 0.7 Gyr best fits the observed data, in particular the presence of the young, metal-poor sequence. Our best-fitting model further implies that the amount of accreted gas in the late-time accretion event needs to be about three times the local gas reservoir in the outer disc at the time of accretion in order to sufficiently dilute the metal abundance. Given this large fraction, we interpret the late-time accretion event as a minor merger presumably with a gas-rich dwarf galaxy with a mass M<10M and a gas fraction of ∼75 per cent.
Original languageEnglish
Pages (from-to)2561-2575
Number of pages15
JournalMonthly Notices of the Royal Astronomical Society
Volume494
Issue number2
Early online date4 Apr 2020
DOIs
Publication statusPublished - 1 May 2020

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    Rights statement: This article has been accepted for publication in MNRAS © 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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