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Co-formation of the disc and the stellar halo★.

Belokurov, V. and Erkal, D. and Evans, N W. and Koposov, S. E. and Deason, A. J. (2018) 'Co-formation of the disc and the stellar halo★.', Monthly notices of the Royal Astronomical Society., 478 (1). pp. 611-619.

Abstract

Using a large sample of main sequence stars with 7D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ∼10 kpc from the Sun) stellar halo. We demonstrate that the halo’s velocity ellipsoid evolves strongly with metallicity. At the low-[Fe/H] end, the orbital anisotropy (the amount of motion in the radial direction compared with the tangential one) is mildly radial, with 0.2 <β< 0.4. For stars with [Fe/H] > −1.7, however, we measure extreme values of β∼ 0.9. Across the metallicity range considered, namely−3 < [Fe/H] < −1, the stellar halo’s spin is minimal, at the level of 20<v¯θ(kms−1)<30 . Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo was deposited in a major accretion event by a satellite with Mvir > 1010M⊙ around the epoch of the Galactic disc formation, between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialization of the massive progenitor’s orbit, amplified by the action of the growing disc.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1093/mnras/sty982
Publisher statement:This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:17 April 2018
Date deposited:27 June 2018
Date of first online publication:02 June 2018
Date first made open access:27 June 2018

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