A Swing of the Pendulum: The Chemodynamics of the Local Stellar Halo Indicate Contributions from Several Radial Merger Events

Abstract

We find that the chemical abundances and dynamics of APOGEE and GALAH stars in the local stellar halo are inconsistent with a scenario in which the inner halo is primarily composed of debris from a single, massive, ancient merger event, as has been proposed to explain the Gaia-Enceladus/Gaia Sausage (GSE) structure. The data contains trends of chemical composition with energy which are opposite to expectations for a single massive, ancient merger event, and multiple chemical evolution paths with distinct dynamics are present. We use a Bayesian Gaussian mixture model regression algorithm to characterize the local stellar halo, and find that the data is best fit by a model with four components. We interpret these components as the VRM, Cronus, Nereus, and Thamnos; however, Nereus and Thamnos likely represent more than one accretion event because the chemical abundance distributions of their member stars contain many peaks. Although the Cronus and Thamnos components have different dynamics, their chemical abundances suggest they may be related. We show that the distinct low and high alpha halo populations from Nissen & Schuster (2010) are explained by VRM and Cronus stars, as well as some in-situ stars. Because the local stellar halo contains multiple substructures, different popular methods of selecting GSE stars will actually select different mixtures of these substructures, which may change the apparent chemodynamic properties of the selected stars. We also find that the Splash stars in the solar region are shifted to higher v_phi and slightly lower [Fe/H] than previously reported.

References in corpus (82)

• Scikit-learn: Machine Learning in Python
• Astropy: A Community Python Package for Astronomy
• The Structure of Cold Dark Matter Halos
• The Gaia mission
• Binary companions of evolved stars in APOGEE DR14: Search method and catalog of ~5,000 companions
• Local Kinematics and the Local Standard of Rest
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe
• galpy: A Python Library for Galactic Dynamics
• The Milky Way Tomography with SDSS: I. Stellar Number Density Distribution
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• The mass distribution and gravitational potential of the Milky Way
• A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: I. Morphology of the Sagittarius Core and Tidal Arms
• Co-formation of the Galactic disc and the stellar halo
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• A geometric distance measurement to the Galactic Center black hole with 0.3% uncertainty
• The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies
• The Ghost of Sagittarius and Lumps in the Halo of the Milky Way
• ASPCAP: The Apogee Stellar Parameter and Chemical Abundances Pipeline
• De Re Metallica: The cosmic chemical evolution of galaxies
• The GALAH+ Survey: Third Data Release
• Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way
• Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• Great Circle tidal streams: evidence for a nearly spherical massive dark halo around the Milky Way
• Building up the Stellar Halo of the Galaxy
• Gaia Early Data Release 3 -- Catalogue validation
• Streams, substructures and the early history of the Milky Way
• Identification of A-colored Stars and Structure in the Halo of the Milky Way from SDSS Commissioning Data
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• Sinking Satellites and the Heating of Galaxy Discs
• The Milky Way stellar halo out to 40 kpc : Squashed, broken but smooth
• The Biggest Splash
• The birth of the Milky Way as uncovered by accurate stellar ages with Gaia
• Kraken reveals itself -- the merger history of the Milky Way reconstructed with the E-MOSAICS simulations
• The astrometric core solution for the Gaia mission. Overview of models, algorithms and software implementation
• Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history
• The QUEST RR Lyrae Survey: Confirmation of the Clump at 50 kpc and Other Over-Densities in the Outer Halo
• Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy
• Using chemical tagging to redefine the interface of the Galactic disk and halo
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• Broken and unbroken: the Milky Way and M31 stellar haloes
• The Hercules-Aquila Cloud
• Inflow, Outflow, Yields, and Stellar Population Mixing in Chemical Evolution Models
• Charting the Galactic acceleration field I. A search for stellar streams with Gaia DR2 and EDR3 with follow-up from ESPaDOnS and UVES
• The Global Dynamical Atlas of the Milky Way mergers: Constraints from Gaia EDR3 based orbits of globular clusters, stellar streams and satellite galaxies
• Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
• Gaia Data Release 2: processing of the photometric data
• The dominant epoch of star formation in the Milky Way formed the thick disc
• APOGEE Chemical Abundance Patterns of the Massive Milky Way Satellites
• Ages and kinematics of chemically selected, accreted Milky Way halo stars
• Apocenter Pile-Up: Origin of the Stellar Halo Density Break
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• Sausage & Mash: The dual origin of the Galactic thick disc and halo from the gas-rich Gaia-Enceladus-Sausage merger
• The SkyMapper-Gaia RVS view of the Gaia-Enceladus-Sausage -- an investigation of the metallicity and mass of the Milky Way's last major merger
• Timing the Early Assembly of the Milky Way with the H3 Survey
• Gaia DR2 reveals a star formation burst in the disc 2-3 Gyr ago
• The imprint of clump formation at high redshift. I. A disc alpha-abundance dichotomy
• The GALAH Survey: Chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and $Gaia$ eDR3
• Principal Component Analysis on Chemical Abundances Spaces
• The GALAH survey: The data reduction pipeline
• The Gaia DR3 view of dynamical substructure in the stellar halo near the Sun
• Broken into Pieces: ATLAS and Aliqa Uma as One Single Stream
• The metal-poor Knee in the Fornax Dwarf Spheroidal Galaxy
• Common origin for Hercules-Aquila and Virgo Clouds in Gaia DR2
• The Stellar Halo of the Galaxy is Tilted & Doubly Broken
• The Progenitors of the Milky Way Stellar Halo: Big Bricks Favoured over Little Bricks
• Co-formation of the thin and thick discs revealed by APOGEE-DR16 and Gaia-DR2
• The Splash without a merger
• A Relationship Between Stellar Metallicity Gradients and Galaxy Age in Dwarf Galaxies
• The Kinematic Properties of Milky Way Stellar Halo Populations
• The homogeneity of the star forming environment of the Milky Way disk over time
• The Pristine survey -- XVI. The metallicity of 26 stellar streams around the Milky Way detected with the STREAMFINDER in Gaia EDR3
• The Milky Way's Shell Structure Reveals the Time of a Radial Collision
• Geometric properties of galactic discs with clumpy episodes
• The Local Stellar Halo is Not Dominated by a Single Radial Merger Event
• Resonant Trapping in the Galactic Disc and Halo and its Relation with Moving Groups
• Age-Dating Red Giant Stars Associated with Galactic Disk and Halo Substructures
• Stars made in outflows may populate the stellar halo of the Milky Way
• The Virgo Overdensity Explained
• Stars in the Local Galactic Thick Disk and Halo in Gaia EDR3: A catalogue of Half a Million Local Main-sequence Stars with Photometric Metallicities
• Mapping Milky Way Halo Substructure using Stars in the Extended Blue Tail of the Horizontal Branch

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• Galactic Archaeology with Gaia
• Nitrogen enrichment and clustered star formation at the dawn of the Galaxy
• The distribution of globular clusters in kinematic spaces does not trace the accretion history of the host galaxy
• Mapping the anisotropic Galactic stellar halo with Blue Horizontal Branch stars
• Metallicity distributions of halo stars: do they trace the Galactic accretion history?
• Exploring the diversity and similarity of radially anisotropic Milky Way-like stellar haloes: implications for disrupted dwarf galaxy searches
• The GALAH survey: Tracing the Milky Way's formation and evolution through RR Lyrae stars
• Four-hundred Very Metal-poor Stars Studied with LAMOST and Subaru. III. Dynamically Tagged Groups and Chemodynamical Properties
• Galaxy archaeology for wet mergers: Globular cluster age distributions in the Milky Way and nearby galaxies
• HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra
• Deciphering the Kinematic Substructure of Local Dark Matter with LAMOST K Giants
• The weak connection between the stellar haloes and merger histories of Milky Way-mass galaxies
• Two faces of Gaia-Sausage-Enceladus: Mining the chemical abundance space with graph attention networks