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The Halo’s Ancient Metal-Rich Progenitor Revealed with BHB Stars

Lancaster, Lachlan; Koposov, Sergey E; Belokurov, Vasily; Evans, N Wyn; Deason, Alis J

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Lachlan Lancaster

Sergey E Koposov

Vasily Belokurov

N Wyn Evans


Using the data from the Sloan Digital Sky Survey and the Gaia satellite, we assemble a pure sample of ∼3000 Blue Horizontal Branch (BHB) stars with 7-D information, including positions, velocities and metallicities. We demonstrate that, as traced with BHBs, the Milky Way’s stellar halo is largely unmixed and can not be well represented with a conventional Gaussian velocity distribution. A single-component model fails because the inner portions of the halo are swamped with metal-rich tidal debris from an ancient, head-on collision, known as the “Gaia Sausage”. Motivated by the data, we build a flexible mixture model which allows us to track the evolution of the halo make-up across a wide range of radii. It is built from two components, one representing the radially anisotropic Sausage stars with their lobed velocity distribution, the other representing a more metal-poor and more isotropic component built up from minor mergers. We show that inside 25 kpc the “Sausage” contributes at least 50% of the Galactic halo. The fraction of “Sausage” stars diminishes sharply beyond 30 kpc, which is the long-established break radius of the classical stellar halo.


Lancaster, L., Koposov, S. E., Belokurov, V., Evans, N. W., & Deason, A. J. (2019). The Halo’s Ancient Metal-Rich Progenitor Revealed with BHB Stars. Monthly Notices of the Royal Astronomical Society, 486(1), 378-389.

Journal Article Type Article
Acceptance Date Mar 18, 2019
Online Publication Date Mar 27, 2019
Publication Date Jun 30, 2019
Deposit Date Apr 4, 2019
Publicly Available Date Apr 11, 2019
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 486
Issue 1
Pages 378-389


Published Journal Article (1.7 Mb)

Copyright Statement
© 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.

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