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.

arXiv commentMNRAS, 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

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• Searching Extra-tidal Features around the Globular Cluster Whiting 1
• Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in the Galaxy
• Voyage 2050 White Paper: All-Sky Visible and Near Infrared Space Astrometry
• Euclid: Early Release Observations -- The surface brightness and colour profiles of the far outskirts of galaxies in the Perseus cluster
• Infrared photometry and CaT spectroscopy of globular cluster M 28 (NGC 6626)
• No globular cluster progenitors in Milky Way satellite galaxies
• Exploring the ex-situ components within Gaia DR3
• Probing the Galactic halo with RR Lyrae stars -- IV. On the Oosterhoff dichotomy of RR Lyrae stars
• Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal) -- III. Age and metallicity distribution of Gaia-Sausage-Enceladus stars near the Sun
• Constraining the Planet Occurrence Rate around Halo Stars of Potentially Extragalactic Origin
• Chemodynamics of metal-poor wide binaries in the Galactic halo: Association with the Sequoia event
• Comparison of the Orbital Properties of the Milky Way Globular Clusters from the Data of the Gaia DR2 and EDR3 Catalogs
• First measurement of the triaxiality of the inner dark matter halo of the Milky Way
• A Ghost in Boötes: The Least Luminous Disrupted Dwarf Galaxy
• HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra
• The global structure of the Milky Way's stellar halo based on the orbits of local metal-poor stars
• Discovery of a pair of very metal-poor stars enriched in neutron-capture elements: The proto-disk r-II star BPS CS 29529-0089 and the Gaia-Sausage-Enceladus r-I star TYC 9219-2422-1
• The Pristine survey XXVIII. Journey to the Galactic outskirts: Mapping the outer halo red giant stars down to the very metal-poor end
• Determination of Sodium Abundance Ratio from Low-Resolution Stellar Spectra and Its Applications
• J-PLUS: Detecting and studying extragalactic globular clusters -- the case of NGC 1023
• A revised view of the Canis Major stellar overdensity with DECam and Gaia: new evidence of a stellar warp of blue stars
• Revealing the Milky Way's Most Recent Major Merger with a Gaia EDR3 Catalog of Machine-Learned Line-of-Sight Velocities
• Nearby stellar substructures in the Galactic halo from DESI Milky Way Survey Year 1 Data Release
• Introducing a new multi-particle collision method for the evolution of dense stellar systems. Crash-test N-body simulations
• Magnesium isotope ratios in Milky Way and dwarf galaxy stars
• The formation and disruption of globular cluster populations in simulations of present-day $L^{\ast }$ galaxies with controlled assembly histories
• On the Origin of the Globular Cluster FSR 1758
• MINCE III. Detailed chemical analysis of the UVES sample
• Dynamical mirages: how bar-induced resonant trapping can mimic substructure clustering in dynamical parameter spaces
• Different Sodium enhancements among multiple populations of Milky Way globular clusters
• The secular evolution of a uniform density star cluster immersed in a compressible galactic tidal field
• Discovery of the shell structure via break radii in the outer halo of the Milky Way
• The neutron-capture and alpha-elements abundance ratios scatter in old stellar populations. Cosmological simulations of the stellar halo
• Survivors and Zombies: The Quenching and Disruption of Satellites around Milky Way Analogs
• Fundamental limits to orbit reconstruction due to non-conservation of stellar actions in a Milky Way-like simulation
• Assessing Uncertainties in the Predicted Very-High-Energy Flux of Globular Clusters in the Cherenkov Telescope Array Era
• Kinematic origin of white dwarfs in the solar neighborhood
• Orbital phase-driven biases in Galactic mass constraints from stellar streams
• Globular clusters in \textsc{OrbIT}: complete dynamical characterisation of the globular cluster population of the Milky Way through updated orbital reconstruction
• Chemical tagging with APOGEE, MUSE, and HST: constraints on the formation of $\omega$ Centauri
• A slow spin to win: The gradual kinematic evolution across metallicities of the proto-Galaxy to the high-$\alpha$ disc
• J01020100-7122208: an accreted evolved blue straggler that wasn't ejected from a supermassive black hole
• The R-Process Alliance: Exploring the cosmic scatter among ten r-process sites with stellar abundances
• Searching for new Hypervelocity Stars with Gaia DR3 and VLT/FORS2 Spectroscopy
• Very metal-poor stars in the solar vicinity: kinematics and abundance analysis
• Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17
• Stellar substructures in the Galactic disc and halo: Properties, origins, and evolution
• The accreted Galaxy: An overview of TESS metal-poor accreted stars candidates
• Globular clusters as indicators of Galactic evolution
• Partitioning the Galactic Halo with Gaussian Mixture Models
• The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar
• The Nephele ecosystem: stars, globular clusters, and stellar streams associated with the progenitor galaxy of $\omega$ Centauri
• Identifying substructure associations in the Milky Way halo using chemo-kinematic tagging
• Chemical abundances of field halo stars -- Implications for the building blocks of the Milky Way
• Main-sequence Turnoff Stars as Probes of the Ancient Galactic Relic: Chemo-dynamical Analysis of a Pilot Sample
• Disentanglement of the chemodynamical assembly: mapping the Milky Way disks
• The metal-poorest tail of the Galactic halo: hypothesis on its origin from precise spectral analysis
• Predicting Images for the Dynamics Of stellar Clusters (π-DOC): a deep learning framework to predict mass, distance and age of globular clusters
• HR-GO II: chemical abundances of low-$E$ retrograde dynamically-tagged-groups: Revealing Thamnos as a very metal-poor substructure
• New families in our Solar neighborhood: applying Gaussian Mixture models for objective classification of structures in the Milky Way and in simulations
• Towards Understanding the Milky Way's Typicality: Assessing the Chemodynamics of M31's Bulge & Bar, Thick & Thin Discs
• Gaia DR3 high radial velocity stars: Genuine fast-moving objects or outliers?
• Rise of the forsaken relics: connecting present-day stellar streams and phase-mixed galaxies to the Epoch of Reionization
• Milky Way globular cluster dynamics: are they preferentially co-rotating?
• Chemo-dynamical reconstruction of Milky Way globular cluster progenitors: Age-metallicity relations and the universality of multiple stellar populations
• A Sequoia stellar candidate with very high 7Li and 9Be
• Searching for stars ejected from the Galactic Centre in DESI
• HAYDN -- High-precision AsteroseismologY of DeNse stellar fields (ESA Voyage 2050 White Paper)
• Using Gaia for studying Milky Way star clusters
• Disruption of a Giant: Spectroscopic Identification of Members in the Periphery and Tidal Tails of $\omega$ Centauri
• A new member of the Milky Way's family tree: Characterizing the Pontus merger of our Galaxy
• A search for stellar structures around nine outer halo globular clusters in the Milky Way
• Are disks of satellites comprised of tidal dwarf galaxies?
• An ancient system hidden in the Galactic plane?
• A relic from a past merger event in the Large Magellanic Cloud
• Purveyors of fine halos III. Chemical abundance analysis of a potential omega Cen associate
• Probing the Milky Way Halo with RR Lyrae Stars from Gaia Data Release 3
• Neutron-capture elements record the ordered chemical evolution of the disc over time
• Gaia-Sausage-Enceladus: Lithium evolution from early red-giant-branch and main-sequence stars
• Searching for and characterizing halo substructures with the GALAH DR4 survey
• The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue -- I. A golden thread through time and space
• Globular clusters of the Gaia Enceladus/Sausage -- I. Orbital and dynamical evolution on cosmological timescales
• APOGEE chemical abundances of stars in the MW satellites Fornax, Sextans, Draco and Carina
• Evolution of action-space coherence in a Milky Way-like simulation