The Milky Way has no in-situ halo other than the heated thick disc. Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE

Abstract

Previous studies based on the analysis of Gaia DR2 data have revealed that accreted stars, possibly originating from a single progenitor satellite, are a significant component of the halo of our Galaxy, potentially constituting most of the halo stars at $[Fe/H]<-1$ within a few kpc from the Sun and beyond. In this paper, we couple astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterize the kinematics and chemistry of in-situ and accreted populations up to $[Fe/H]\sim -2$. Accreted stars appear to significantly impact the Galactic chemo-kinematic relations, not only at $[Fe/H]<-1$, but also at metallicities typical of the thick and metal-poor thin discs. They constitute about 60% of all stars at $[Fe/H]<-1$, the remaining 40% being made of (metal-weak) thick disc stars. We find that the stellar kinematic fossil record shows the imprint left by this accretion event which heated the old Galactic disc. We are able to age-date this kinematic imprint, showing that the accretion occurred between 9 and 11 Gyr ago, and that it led to the last significant heating of the Galactic disc. An important fraction of stars with abundances typical of the (metal-rich) thick disc, and heated by this interaction, is now found in the Galactic halo. Indeed about half of the kinematically defined halo at few kpc from the Sun is composed of metal-rich thick disc stars. Moreover, we suggest that this metal-rich thick disc component dominates the stellar halo of the inner Galaxy. The new picture that emerges from this study is one where the standard non-rotating in-situ halo population, the collapsed halo, seems to be more elusive than ever.

arXiv comment25 pages, 24 figures, accepted for publication on A&A

References in corpus (82)

• Gaia Data Release 2. Summary of the contents and survey properties
• The Gaia mission
• Gaia Data Release 2: The astrometric solution
• Local Kinematics and the Local Standard of Rest
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• Trigonometric Parallaxes of High Mass Star Forming Regions: the Structure and Kinematics of the Milky Way
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• Co-formation of the Galactic disc and the stellar halo
• Stellar Chemical Signatures And Hierarchical Galaxy Formation
• Kinematics of Metal-Poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Non-Kinematically Selected Stars
• Two Stellar Components in the Halo of the Milky Way
• Gaia Data Release 2: Catalogue validation
• Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way
• Chemical abundances of 1111 FGK stars from the HARPS GTO planet search program.Galactic stellar populations and planets
• Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
• The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk
• Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from SDSS DR7
• The spatial structure of mono-abundance sub-populations of the Milky Way disk
• The accretion origin of the Milky Way's stellar halo
• Quantifying the Fragility of Galactic Disks in Minor Mergers
• Tracing Galaxy Formation with Stellar Halos II: Relating Substructure in Phase- and Abundance-Space to Accretion Histories
• Gaia Data Release 1: Catalogue validation
• The formation and assembly history of the Milky Way revealed by its globular cluster population
• In disguise or out of reach: first clues about in-situ and accreted stars in the stellar halo of the Milky Way from Gaia DR2
• Sinking Satellites and the Heating of Galaxy Discs
• Cold Dark Matter Substructure and Galactic Disks I: Morphological Signatures of Hierarchical Satellite Accretion
• Cosmological simulations of the formation of the stellar haloes around disc galaxies
• The origin of accreted stellar halo populations in the Milky Way using APOGEE, $\text{Gaia}$, and the EAGLE simulations
• Radial mixing and the transition between the thick and thin Galactic discs
• The Bifurcated Age-Metallicity Relation of Milky Way Globular Clusters and its Implications For the Accretion History of the Galaxy
• The stellar population structure of the Galactic disk
• Mapping the substructure in the Galactic halo with the next generation of astrometric satellites
• A First Constraint on the Thick Disk Scale Length: Differential Radial Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc
• The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach
• Formation and Evolution of the Disk System of the Milky Way: [alpha/Fe] Ratios and Kinematics of the SEGUE G-Dwarf Sample
• Using chemical tagging to redefine the interface of the Galactic disk and halo
• The Sausage Globular Clusters
• Gaia reveals a metal-rich in-situ component of the local stellar halo
• Ghostly Tributaries to the Milky Way: Charting the Halo's Stellar Streams with the Gaia DR2 catalogue
• Reconstructing the star formation history of the Milky Way disc(s) from chemical abundances
• Two distinct halo populations in the solar neighborhood. II. Evidence from stellar abundances of Mn, Cu, Zn, Y, and Ba
• One large blob and many streams frosting the nearby stellar halo in Gaia DR2
• Building Late-Type Spiral Galaxies by In-Situ and Ex-Situ Star Formation
• Quantifying Kinematic Substructure in the Milky Way's Stellar Halo
• The dominant epoch of star formation in the Milky Way formed the thick disc
• Global structure and kinematics of stellar haloes in cosmological hydrodynamic simulations
• Disentangling the Galactic Halo with APOGEE: I. Chemical and Kinematical Investigation of Distinct Metal-Poor Populations
• The shape of the Galactic halo with $Gaia$ DR2 RR Lyrae. Anatomy of an ancient major merger
• The origin of galactic metal-rich stellar halo components with highly eccentric orbits
• A Short Scale Length for the α-Enhanced Thick Disk of the Milky Way: Evidence from Low-Latitude SEGUE Data
• Phase-Space Distributions of Chemical Abundances in Milky Way-type Galaxy Halos
• Off the beaten path: Gaia reveals GD-1 stars outside of the main stream
• Two distinct halo populations in the solar neighborhood. III. Evidence from stellar ages and orbital parameters
• Tracing the formation of the Milky Way through ultra metal-poor stars
• Can Gas prevent the Destruction of Thin Stellar Discs by Minor Mergers?
• Heating of Galactic Disks by Infalling Satellites
• On the kinematic detection of accreted streams in the Gaia era: a cautionary tale
• The Dual Origin of Stellar Halos II: Chemical Abundances as Tracers of Formation History
• Halo Star Streams in the Solar Neighborhood
• Oxygen abundances in low- and high-alpha field halo stars and the discovery of two field stars born in globular clusters
• Gaia Data Release 1. Testing the parallaxes with local Cepheids and RR Lyrae stars
• On the identification of merger debris in the {\it Gaia} Era
• Metal Abundances and Kinematics of Bright Metal-Poor Giants Selected from the LSE Survey: Implications for the Metal-Weak Thick Disk
• A new stellar chemo-kinematic relation reveals the merger history of the Milky Way disc
• Halo Substructure and Disk Heating in a Lambda CDM Universe
• In the thick of it: metal-poor disc stars in RAVE
• Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths
• Disc Heating: Comparing the Milky Way with Cosmological Simulations
• Common origin for Hercules-Aquila and Virgo Clouds in Gaia DR2
• The Century Survey Galactic Halo Project III: A Complete 4300 deg^2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo
• Searching for the metal-weak thick disc in the solar neighbourhood
• Simulations of minor mergers. II. The phase-space structure of thick discs
• The Gaia-ESO Survey: New constraints on the Galactic disc velocity dispersion and its chemical dependencies
• Phlegethon, a nearby $75°$-long retrograde stellar stream
• Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies
• The Milky Way as a High Redshift Galaxy: The Importance of Thick Disk Formation in Galaxies
• The slowing down of galaxy disks in dissipationless minor mergers
• Characteristics of the two sequences seen in the high-velocity Hertzsprung-Russell diagram in Gaia DR2
• Chemical Mapping of the Milky Way With The Canada-France Imaging Survey: A Non-parametric Metallicity-Distance Decomposition of the Galaxy
• The assembly history of the Galactic inner halo inferred from alpha-patterns
• The metal-rich halo component extended in z: a characterization with Gaia DR2 and APOGEE
• The Gaia-ESO Survey: α-abundances of metal-poor stars

Cited by in corpus (153)

• Streams, substructures and the early history of the Milky Way
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• The Biggest Splash
• The birth of the Milky Way as uncovered by accurate stellar ages with Gaia
• A time-resolved picture of our Milky Way's early formation history
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation
• Age dissection of the Milky Way discs: red giants in the Kepler field
• Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way's quiescent merger history and in-situ bulge
• Dynamical Relics of the Ancient Galactic Halo
• Chronologically dating the early assembly of the Milky Way
• Sausage & Mash: The dual origin of the Galactic thick disc and halo from the gas-rich Gaia-Enceladus-Sausage merger
• The Poor Old Heart of the Milky Way
• Resolving the Metallicity Distribution of the Stellar Halo with the H3 Survey
• 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
• StarHorse results for spectroscopic surveys + Gaia DR3: Chrono-chemical populations in the solar vicinity, the genuine thick disk, and young-alpha rich stars
• Weighing the stellar constituents of the Galactic halo with APOGEE red giant stars
• Chemo-kinematics of the $Gaia$ RR Lyrae: the halo and the disc
• Selecting accreted populations: metallicity, elemental abundances, and ages of the Gaia-Sausage-Enceladus and Sequoia populations
• Gaia Early Data Release 3: The Galactic anticentre
• The GALAH Survey: Chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and $Gaia$ eDR3
• Reconstructing the Disrupted Dwarf Galaxy $Gaia$-Sausage/Enceladus Using its Stars and Globular Clusters
• Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies
• VINTERGATAN II: the history of the Milky Way told by its mergers
• Stellar migrations and metal flows -- Chemical evolution of the thin disc of a simulated Milky Way analogous galaxy
• Galactic Archaeology with Gaia
• Chemo-dynamics of outer halo dwarf stars, including \textit{Gaia}-Sausage and \textit{Gaia}-Sequoia candidates
• $R$-process enhancements of Gaia-Enceladus in GALAH DR3
• GASTRO library I: the simulated chemodynamical properties of several GSE-like stellar halos
• Bimodality of [α/Fe]-[Fe/H] distributions is a natural outcome of dissipative collapse and disc growth in Milky Way-type galaxies
• The GALAH Survey: Chemodynamics of the Solar Neighbourhood
• Reviving old controversies: is the early Galaxy flat or round? Investigations into the early phases of the Milky Way formation through stellar kinematics and chemical abundances
• Gaia Data Release 3: Chemical cartography of the Milky Way
• Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys
• VINTERGATAN III: how to reset the metallicity of the Milky Way
• Merger-induced galaxy transformations in the ARTEMIS simulations
• The Splash without a merger
• Chasing the impact of the Gaia-Sausage-Enceladus merger on the formation of the Milky Way thick disc
• Constraints on the assembly history of the Milky Way's smooth, diffuse stellar halo from the metallicity-dependent, radially-dominated velocity anisotropy profiles probed with K giants and BHB stars using LAMOST, SDSS/SEGUE, and Gaia
• The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2
• The Kinematic Properties of Milky Way Stellar Halo Populations
• Extreme kinematic misalignment in IllustrisTNG galaxies: the origin, structure and internal dynamics of galaxies with a large-scale counterrotation
• Chemo-kinematics of the Milky Way spiral arms and bar resonances: connection to ridges and moving groups in the Solar vicinity
• Substructure in the stellar halo near the Sun. II. Characterisation of independent structures
• Substructure in the stellar halo near the Sun. I. Data-driven clustering in Integrals of Motion space
• The distribution of globular clusters in kinematic spaces does not trace the accretion history of the host galaxy
• Energy wrinkles and phase-space folds of the last major merger
• A Blueprint for the Milky Way's Stellar Populations: The Power of Large Photometric and Astrometric Surveys
• Abundances in the Milky Way across five nucleosynthetic channels from 4 million LAMOST stars
• The stellar velocity distribution function in the Milky Way galaxy
• High-precision chemical abundances of Galactic building blocks. The distinct chemical abundance sequence of Sequoia
• Mapping the Galactic disk with the LAMOST and $Gaia$ Red clump sample: V: On the origin of the "young" [$\alpha$/Fe]-enhanced stars
• Radial structure and formation of the Milky Way disc
• On the origin of a rotating metal-poor stellar population in the Milky Way Nuclear Cluster
• Dynamically Tagged Groups of Metal-Poor Stars from the Best \& Brightest Survey
• The Milky Way's bulge star formation history as constrained from its bimodal chemical abundance distribution
• Neutron star mergers as the astrophysical site of the r-process in the Milky Way and its satellite galaxies
• The Pristine survey XIII: Uncovering the very metal-poor tail of the thin disc
• The impact of early massive mergers on the chemical evolution of Milky Way-like galaxies: insights from NIHAO-UHD simulations
• Cluster Ages to Reconstruct the Milky Way Assembly (CARMA) I. The final word on the origin of NGC6388 and NGC6441
• Local RR Lyrae Stars: native and alien
• Discovery of the local counterpart of disc galaxies at z > 4: The oldest thin disc of the Milky Way using Gaia-RVS
• Stars made in outflows may populate the stellar halo of the Milky Way
• Icarus: a Flat and Fast Prograde Stellar Stream in the Milky Way disk
• A bottom-heavy initial mass function for the likely-accreted blue-halo stars of the Milky Way
• Deciphering the evolution of the Milky Way discs: Gaia APOGEE Kepler giant stars and the Besançon Galaxy Model
• Origin of highly $r$-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy
• The chemodynamics of prograde and retrograde Milky Way stars
• Reading the CARDs: the Imprint of Accretion History in the Chemical Abundances of the Milky Way's Stellar Halo
• VINTERGATAN-GM: The cosmological imprints of early mergers on Milky-Way-mass galaxies
• The R-Process Alliance: Chemo-Dynamically Tagged Groups II. An Extended Sample of Halo $r$-Process-Enhanced Stars
• Dynamically Tagged Groups of Metal-Poor Stars II. The Radial Velocity Experiment Data Release 6
• The stellar halo in Local Group Hestia simulations I. The in-situ component and the effect of mergers
• Metallicity distributions of halo stars: do they trace the Galactic accretion history?
• The Unmixed Debris of Gaia-Sausage/Enceladus in the Form of a Pair of Halo Stellar Overdensities
• Targeting Bright Metal-poor Stars in the Disk and Halo Systems of the Galaxy
• The stellar halo in Local Group Hestia simulations II. The accreted component
• The stellar halo in Local Group Hestia simulations III. Chemical abundance relations for accreted and in-situ stars
• The e-TidalGCs Project: Modeling the extra-tidal features generated by Galactic globular clusters
• Chronology of the chemical enrichment of the old Galactic stellar populations
• The nature of the Milky Way's stellar halo revealed by the three integrals of motion
• The AMBRE Project: Solar neighbourhood chemodynamical constraints on Galactic disc evolution
• The first retrieval of a substellar subdwarf: A cloud-free SDSS J125637.13-022452.4
• A Blueprint for the Milky Way's Stellar Populations. III. Spatial Distributions and Population Fractions of Local Halo Stars
• The imprint of clump formation at high redshift. II. The chemistry of the bulge
• Elemental Abundances in M31: Properties of the Inner Stellar Halo
• Death at watersheds: galaxy quenching in low-density environments
• Origin of metals in old Milky Way halo stars based on GALAH and Gaia
• A portrait of the Vast Polar Structure as a young phenomenon: hints from its member satellites
• Probing the early Milky Way with GHOST spectra of an extremely metal-poor star in the Galactic disk
• Linking nearby stellar streams to more distant halo overdensities
• Stragglers of the thick disc
• Dynamics of Stellar Disk Tilting from Satellite Mergers
• The metal-poor end of the Spite plateau. II. Detailed chemical investigation
• Galactic ArchaeoLogIcaL ExcavatiOns (GALILEO) II. t-SNE Portrait of Local Fossil Relics and Structures
• Antaeus: a retrograde group of tidal debris in the Milky Way's disk plane
• Explaining the chemical trajectories of accreted and in-situ halo stars of the Milky Way
• Study of the departures from LTE in the unevolved stars infra-red spectra
• Using Action Space Clustering to Constrain the Accretion History of Milky Way like Galaxies
• A Unified Exploration of the Chronology of the Galaxy
• Exploring the chemodynamics of metal-poor stellar populations
• Exoplanets Across Galactic Stellar Populations with PLATO: Estimating Exoplanet Yields Around FGK Stars for the Thin Disk, Thick Disk and Stellar Halo
• High-speed stars. II. An unbound star, young stars, bulge metal-poor stars, and Aurora candidates
• High Velocity Stars in SDSS/APOGEE DR17
• Chemodynamical Analysis of Metal-rich High-eccentricity Stars in the Milky Way's Disk
• A high fidelity Milky Way simulation with Kraken, Gaia-Enceladus, and Sequoia analogues: clues to their accretion histories
• Evolution of the radial ISM metallicity gradient in the Milky Way disk since redshift $\approx 3$
• The Pristine survey: XXV. The very metal-poor Galaxy: Chemodynamics through the follow-up of the Pristine-Gaia synthetic catalogue
• Connecting the local stellar halo and its dark matter density to dwarf galaxies via blue stragglers
• The heavy-elements heritage of the falling sky
• The Pristine survey -- XX: GTC follow-up observations of extremely metal-poor stars identified from Pristine and LAMOST
• Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal) II. Unveiling the formation and evolution of the kinematically selected Thick and Thin Discs
• Hidden deep in the halo: selection of a reduced proper motion halo catalogue and mining retrograde streams in the velocity space
• Searching for Extragalactic Exoplanetary Systems: the Curious Case of BD+20 2457
• Chemo-dynamics and asteroseismic ages of seven metal-poor red giants from the Kepler field
• Absence of obvious tidal tails around the globular cluster NGC 6397
• Internal calibration of LAMOST and Gaia DR3 GSP-Spec stellar abundances
• COSMOS Brightest Group Galaxies -- III: Evolution of stellar ages
• How well can we unravel the accreted constituents of the Milky Way stellar halo? A test on cosmological hydrodynamical simulations
• Investigating three Sirius-like systems with SPHERE
• The formation history of the Milky Way disc with high-resolution cosmological simulations
• L-GALAXIES 2020: The formation and chemical evolution of stellar haloes in Milky Way Analogues and galaxy clusters
• Rediscovering the Milky Way with orbit superposition approach and APOGEE data I. Method validation
• Rediscovering the Milky Way with orbit superposition approach and APOGEE data II. Chrono-chemo-kinematics of the disc
• Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in the Galaxy
• Bar-spiral interaction produces radial migration and star formation bursts
• Resolved SPLASH Chemodynamics in Andromeda's PHAT Stellar Halo and Disk: On the Nature of the Inner Halo Along the Major Axis
• New stellar age estimates using SPInS based on Gaia DR3 photometry and LAMOST DR8 abundances
• A chemodynamical analysis of bright metal-poor stars from the HESP-GOMPA survey -- Indications of a non-prevailing site for light r-process elements
• HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra
• Revealing the Milky Way's Most Recent Major Merger with a Gaia EDR3 Catalog of Machine-Learned Line-of-Sight Velocities
• Introducing a new multi-particle collision method for the evolution of dense stellar systems. Crash-test N-body simulations
• The Chemodynamical Nature of the Triangulum-Andromeda Overdensity
• Orbital phase-driven biases in Galactic mass constraints from stellar streams
• Investigating the vertical distribution of the disk as a function of radial action
• A slow spin to win: The gradual kinematic evolution across metallicities of the proto-Galaxy to the high-$\alpha$ disc
• The disc origin of the Milky Way bulge: On the high velocity dispersion of metal-rich stars at low latitude
• The role of environment on the evolution of disc galaxies density profiles New insights from simulations and comparison to Euclid data
• Rediscovering the Milky Way with orbit superposition approach and APOGEE data III. Panoramic view of the bulge
• Disentanglement of the chemodynamical assembly: mapping the Milky Way disks
• Partitioning the Galactic Halo with Gaussian Mixture Models
• Build-up and survival of the disc: From numerical models of galaxy formation to the Milky Way
• Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17
• Chronos --- Taking the pulse of our Galactic neighbourhood (ESA Voyage 2050 White Paper)
• Deciphering the Milky Way's star formation at cosmic noon with high proper-motion stars: A precursor to the merger-driven starburst
• HR-GO II: chemical abundances of low-$E$ retrograde dynamically-tagged-groups: Revealing Thamnos as a very metal-poor substructure
• Two faces of Gaia-Sausage-Enceladus: Mining the chemical abundance space with graph attention networks
• Rapid early gas accretion for the inner Galactic disc
• Bar Evolution in Edge-on Galaxies: A Demographic Study of Boxy/Peanut Bulges
• The Milky Way's Early Disk Was Built in Bursts. I. Evidence from Low-angular-momentum, High-$\alpha$ Stars Scattered Out of the Protodisk by Merger Events
• The COMBS Survey -- III. The Chemodynamical Origins of Metal-Poor Bulge Stars
• Probing the Milky Way Halo with RR Lyrae Stars from Gaia Data Release 3
• The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue -- I. A golden thread through time and space