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

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

Abstract

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.

Citation

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. https://doi.org/10.1093/mnras/stz853

• 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
• DOI: https://doi.org/10.1093/mnras/stz853