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Lessons from the Auriga discs : the hunt for the Milky Way's ex situ disc is not yet over.

Gómez, Facundo A. and Grand, Robert J. J. and Monachesi, Antonela and White, Simon D. M. and Bustamante, Sebastian and Marinacci, Federico and Pakmor, Rüdiger and Simpson, Christine M. and Springel, Volker and Frenk, Carlos S. (2017) 'Lessons from the Auriga discs : the hunt for the Milky Way's ex situ disc is not yet over.', Monthly Notices of the Royal Astronomical Society., 472 (3). pp. 3722-3733.


We characterize the contribution from accreted material to the galactic discs of the Auriga Project, a set of high-resolution magnetohydrodynamic cosmological simulations of late-type galaxies performed with the moving-mesh code AREPO. Our goal is to explore whether a significant accreted (or ex situ) stellar component in the Milky Way disc could be hidden within the near-circular orbit population, which is strongly dominated by stars born in situ. One-third of our models shows a significant ex situ disc but this fraction would be larger if constraints on orbital circularity were relaxed. Most of the ex situ material (≳50 per cent) comes from single massive satellites (>6 × 1010 M⊙). These satellites are accreted with a wide range of infall times and inclination angles (up to 85°). Ex situ discs are thicker, older and more metal poor than their in situ counterparts. They show a flat median age profile, which differs from the negative gradient observed in the in situ component. As a result, the likelihood of identifying an ex situ disc in samples of old stars on near-circular orbits increases towards the outskirts of the disc. We show three examples that, in addition to ex situ discs, have a strongly rotating dark matter component. Interestingly, two of these ex situ stellar discs show an orbital circularity distribution that is consistent with that of the in situ disc. Thus, they would not be detected in typical kinematic studies.

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Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:17 August 2017
Date deposited:01 November 2017
Date of first online publication:22 August 2017
Date first made open access:01 November 2017

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