Evidence for the Third Stellar Population in the Milky Way's Disk

Abstract

The Milky Way is a unique laboratory, where stellar properties can be measured and analyzed in detail. In particular, stars in the older populations encode information on the mechanisms that led to the formation of our Galaxy. In this article, we analyze the kinematics, spatial distribution, and chemistry of a large number of stars in the Solar Neighborhood, where all of the main Galactic components are well-represented. We find that the thick disk comprises two distinct and overlapping stellar populations, with different kinematic properties and chemical compositions. The metal-weak thick disk (MWTD) contains two times less metal content than the canonical thick disk, and exhibits enrichment of light elements typical of the oldest stellar populations of the Galaxy. The rotational velocity of the MWTD around the Galactic center is ~ 150 km s^(-1), corresponding to a rotational lag of 30 km s^(-1) relative to the canonical thick disk (~ 180 km s^(-1)), with a velocity dispersion of 60 km s^(-1). This stellar population likely originated from the merger of a dwarf galaxy during the early phases of our Galaxy's assembly, or it is a precursor disk, formed in the inner Galaxy and brought into the Solar Neighborhood by bar instability or spiral-arm formation mechanisms.

arXiv comment25 pages, 10 figures, Accepted for Publication on the Astrophysical Journal

References in corpus (54)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• The Sloan Digital Sky Survey: Technical Summary
• Gaia Data Release 2. Summary of the contents and survey properties
• The Seventh Data Release of the Sloan Digital Sky Survey
• Gaia Data Release 2: The astrometric solution
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• SEGUE: A Spectroscopic Survey of 240,000 stars with g=14-20
• Tracing Galaxy Formation with Stellar Halos I: Methods
• The Universal Stellar Mass-Stellar Metallicity Relation for Dwarf Galaxies
• The Ghost of Sagittarius and Lumps in the Halo of the Milky Way
• Gaia Data Release 2: using Gaia parallaxes
• ASPCAP: The Apogee Stellar Parameter and Chemical Abundances Pipeline
• 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
• Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk
• The Milky Way Tomography with SDSS: II. Stellar Metallicity
• The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• Structure and Kinematics of the Stellar Halos and Thick Disks of the Milky Way Based on Calibration Stars from SDSS DR7
• A database of high and medium-resolution stellar spectra
• The spatial structure of mono-abundance sub-populations of the Milky Way disk
• The Milky Way's circular velocity curve between 4 and 14 kpc from APOGEE data
• Abundances, Stellar Parameters, and Spectra From the SDSS-III/APOGEE Survey
• The Fifteenth Data Release of the Sloan Digital Sky Surveys: First Release of MaNGA Derived Quantities, Data Visualization Tools and Stellar Library
• The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
• One Ring to Encompass them All: A giant stellar structure that surrounds the Galaxy
• The birth of the Milky Way as uncovered by accurate stellar ages with Gaia
• The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
• The Milky Way's Stellar Disk
• The Gaia-ESO Survey: the Galactic Thick to Thin Disc transition
• Globular clusters, satellite galaxies and stellar haloes from early dark matter peaks
• Formation and Evolution of the Disk System of the Milky Way: [alpha/Fe] Ratios and Kinematics of the SEGUE G-Dwarf Sample
• The Case for the Dual Halo of the Milky Way
• Confirmation of the $Gaia$ DR2 parallax zero-point offset using asteroseismology and spectroscopy in the $Kepler$ field
• Simultaneous calibration of spectro-photometric distances and the Gaia DR2 parallax zero-point offset with deep learning
• Constraining the thick disc formation scenario of the Milky Way
• Distances and parallax bias in Gaia DR2
• The SEGUE Stellar Parameter Pipeline. V. Estimation of Alpha-Element Abundance Ratios From Low-Resolution SDSS/SEGUE Stellar Spectra
• Dark matter response to galaxy formation
• A Short Scale Length for the α-Enhanced Thick Disk of the Milky Way: Evidence from Low-Latitude SEGUE Data
• Stellar Streams and Clouds in the Galactic Halo
• Population Studies. XIII. A New Analysis of the Bidelman-MacConnell "Weak-Metal" Stars - Confirmation of Metal-Poor Stars in the Thick Disk of the Galaxy
• Kinematics of the local disk from the RAVE survey and the Gaia first data release
• A Gaia-Enceladus Analog in the EAGLE Simulation: Insights into the Early Evolution of the Milky Way
• Testing systematics of Gaia DR2 parallaxes with empirical surface brightness -- color relations applied to eclipsing binaries
• The fall of the Northern Unicorn: Tangential motions in the Galactic Anti-centre with SDSS and Gaia
• A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities
• Rotating halo traced by the K giant stars from LAMOST and Gaia
• Stellar disc streams as probes of the Galactic potential and satellite impacts
• Gaia DR2 orbital properties for field stars with globular cluster-like CN band strengths
• Chemical and Dynamical Properties of the Stellar Halo

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• Sausage & Mash: The dual origin of the Galactic thick disc and halo from the gas-rich Gaia-Enceladus-Sausage merger
• The Pristine survey X: a large population of low-metallicity stars permeates the Galactic disk
• Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys
• The R-Process Alliance: Chemo-Dynamically Tagged Groups of Halo $r$-Process-Enhanced Stars Reveal a Shared Chemical-Evolution History
• The Atari Disk, a Metal-Poor Stellar Population in the Disk System of the Milky Way
• The Photometric Metallicity and Carbon Distributions of the Milky Way's Halo and Solar Neighborhood from S-PLUS Observations of SDSS Stripe 82
• The COMBS Survey -- II. Distinguishing the Metal-Poor Bulge from the Halo Interlopers
• Elemental Abundances in M31: A Comparative Analysis of Iron and Alpha Element Abundances in the Outer Disk, Giant Stellar Stream, and Inner Halo of M31
• A Blueprint for the Milky Way's Stellar Populations: The Power of Large Photometric and Astrometric Surveys
• Dynamically Tagged Groups of Metal-Poor Stars from the Best \& Brightest Survey
• The Pristine survey XIII: Uncovering the very metal-poor tail of the thin disc
• Origin of highly $r$-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy
• Chemo-Dynamically Tagged Groups of CEMP Stars in the Halo of the Milky Way. I. Untangling the Origins of CEMP-$s$ and CEMP-no Stars
• The R-Process Alliance: Chemo-Dynamically Tagged Groups II. An Extended Sample of Halo $r$-Process-Enhanced Stars
• The formation and survival of the Milky Way's oldest stellar disk
• Dynamically Tagged Groups of Metal-Poor Stars II. The Radial Velocity Experiment Data Release 6
• Targeting Bright Metal-poor Stars in the Disk and Halo Systems of the Galaxy
• Star Formation Timescales of the Halo Populations from Asteroseismology and Chemical Abundances
• The nature of the Milky Way's stellar halo revealed by the three integrals of motion
• A Blueprint for the Milky Way's Stellar Populations. III. Spatial Distributions and Population Fractions of Local Halo Stars
• The Metallicity Gradient and Complex Formation History of the Outermost Halo of the Milky Way
• Candidate Members of the VMP/EMP Disk System of the Galaxy from the SkyMapper and SAGES Surveys
• Four-hundred Very Metal-poor Stars Studied with LAMOST and Subaru. III. Dynamically Tagged Groups and Chemodynamical Properties
• A Blueprint for the Milky Way's Stellar Populations. II. Improved Isochrone Calibration in the SDSS and Pan-STARRS Photometric Systems
• Understanding the early stages of galaxy formation using very metal-poor stars from the Hamburg/ESO survey
• On the metal-poor edge of the Milky Way "thin disc"
• Insights into the Formation and Evolution History of the Galactic Disk System
• Differential rotation of the halo traced by the K-giant stars
• Two Populations of Carbon-Enhanced Metal-Poor Stars in the Disk System of the Milky Way
• Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in the Galaxy
• Deciphering the Milky Way disc formation time encrypted in the bar chrono-kinematics
• Element abundance analysis of the metal-rich stellar halo and high-velocitythick disk in the galaxy
• High-Resolution Chemical Abundances of the Nyx Stream
• Impact of QCD sum rules coupling constants on neutron stars structure
• The global structure of the Milky Way's stellar halo based on the orbits of local metal-poor stars
• Determination of Sodium Abundance Ratio from Low-Resolution Stellar Spectra and Its Applications
• Search for the metal-weak thick disk from the LAMOST DR5
• Unveiling Galactic substructures with M Giant stars: A kinematic and chemical study based on LAMOST DR9, Gaia DR3 and APOGEE DR17
• The COMBS Survey -- III. The Chemodynamical Origins of Metal-Poor Bulge Stars