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 (27)

• The Gaia mission
• galpy: A Python Library for Galactic Dynamics
• The mass distribution and gravitational potential of the Milky Way
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• The Hercules-Aquila Cloud
• 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
• APOGEE Chemical Abundance Patterns of the Massive Milky Way Satellites
• The Fall of a Giant. Chemical evolution of Enceladus, alias the Gaia Sausage
• 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
• The GALAH survey: The data reduction pipeline
• The Gaia DR3 view of dynamical substructure in the stellar halo near the Sun
• The Progenitors of the Milky Way Stellar Halo: Big Bricks Favoured over Little Bricks
• The Stellar Halo of the Galaxy is Tilted & Doubly Broken
• 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
• Geometric properties of galactic discs with clumpy episodes
• The Local Stellar Halo is Not Dominated by a Single Radial Merger Event
• Age-Dating Red Giant Stars Associated with Galactic Disk and Halo Substructures
• 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

Cited by in corpus (13)

• 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