Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo

Abstract

The Gaia Sausage is the major accretion event that built the stellar halo of the Milky Way galaxy. Here, we provide dynamical and chemical evidence for a second substantial accretion episode, distinct from the Gaia Sausage. The Sequoia Event provided the bulk of the high energy retrograde stars in the stellar halo, as well as the recently discovered globular cluster FSR 1758. There are up to 6 further globular clusters, including $\omega$~Centauri, as well as many of the retrograde substructures in Myeong et al. (2018), associated with the progenitor dwarf galaxy, named the Sequoia. The stellar mass in the Sequoia galaxy is $\sim 5\times 10^7{M}_{\odot }$, whilst the total mass is $\sim 10^{10}{M}_{\odot }$, as judged from abundance matching or from the total sum of the globular cluster mass. Although clearly less massive than the Sausage, the Sequoia has a distinct chemo-dynamical signature. The strongly retrograde Sequoia stars have a typical eccentricity of $\sim 0.6$, whereas the Sausage stars have no clear net rotation and move on predominantly radial orbits. On average, the Sequoia stars have lower metallicity by $\sim 0.3$ dex and higher abundance ratios as compared to the Sausage. We conjecture that the Sausage and the Sequoia galaxies may have been associated and accreted at a comparable epoch.

MNRAS, revised version in light of referee's comments. Minor modifications and clarifications to the argument, as well as an updated list of Sausage Globular Clusters

References in corpus (17)

• The EAGLE project: Simulating the evolution and assembly of galaxies and their environments
• The mass distribution and gravitational potential of the Milky Way
• Two Stellar Components in the Halo of the Milky Way
• The Auriga Project: the properties and formation mechanisms of disc galaxies across cosmic time
• Detection of a 63 Degree Cold Stellar Stream in the Sloan Digital Sky Survey
• A systematic survey for infrared star clusters with |b|<20deg using 2MASS
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• Dark Matter Halos in Galaxies and Globular Cluster Populations
• A box full of chocolates: The rich structure of the nearby stellar halo revealed by Gaia and RAVE
• Halo Substructure in the SDSS-Gaia Catalogue : Streams and Clumps
• Origin of the excess of high-energy retrograde stars in the Galactic halo
• A Sequoia in the Garden: FSR 1758 - Dwarf Galaxy or Giant Globular Cluster?
• The chemical evolution of Omega Centauri's progenitor system
• Modeling the chemical evolution of Omega Centauri using three-dimensional hydrodynamical simulations
• The retrograde orbit of the globular cluster FSR1758 revealed with Gaia DR2
• The Magellanic Group and the Seven Dwarfs
• Identification of the Long Stellar Stream of the Prototypical Massive Globular Cluster $\omega$ Centauri

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• The Pristine survey VIII: The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime
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• The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2
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• How Unusual is the Milky Way's Assembly History?
• Local RR Lyrae Stars: native and alien
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• The Most Metal-poor Stars in Omega Centauri (NGC 5139)
• Discovery of new globular clusters in the Sagittarius dwarf galaxy
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• Constraints on the Galactic Inner Halo Assembly History from the Age Gradient of Blue Horizontal-branch Stars
• The HR 1614 moving group is not a dissolving cluster
• Dependence of Galactic Halo Kinematics on the Adopted Galactic Potential
• Connecting the local stellar halo and its dark matter density to dwarf galaxies via blue stragglers
• Differential rotation of the halo traced by the K-giant stars
• An outer shade of Pal: Abundance analysis of the outer halo globular cluster Palomar 13
• Infrared photometry and CaT spectroscopy of globular cluster M 28 (NGC 6626)
• Predicting Images for the Dynamics Of stellar Clusters (π-DOC): a deep learning framework to predict mass, distance and age of globular clusters