The nature of the Milky Way's stellar halo revealed by the three integrals of motion

Abstract

We developed a new selection method of halo stars in the phase-space distribution defined by the three integrals of motion in an axisymmetric Galactic potential, ($E$, $L_z$, $I_3$), where $I_3$ is the third integral of motion. The method is used to explore the general chemo-dynamical structure of the halo based on stellar samples from SDSS-SEGUE DR7 and DR16-APOGEE, matched with Gaia-DR2. We found, (a) halo stars can be separated from disk stars by selecting over (1) $0<L_z<1500$ \kpckms, $(2I_3{)}^{1/2}>1000$ \kpckms (orbital angle ${\theta }_{\mathrm{orb}}$ $>$ 15-20 deg), and $E<-1.5\times 10^5$ km${}^2$ s${}^{-2}$, and (2) $L_z<0$ \kpckms. These selection criteria are free from kinematical biases introduced by the simple high-velocity cuts adopted in recent literature; (b) the averaged, or {\it coarse-grained}, halo phased-space distribution shows a monotonic exponential decrease with increasing $E$ and $I_3$ like the Michie-Bodenheimer models; (c) the inner stellar halo described in \citet{carollo2007,carollo2010} is found to comprise a combination of Gaia Enceladus debris (GE; \citealt{helmi2018}), lowest-$E$ stars (likely in-situ stars), as well as metal-poor prograde stars missed by the high velocity cuts selection; (d) the very metal poor outer halo, ([Fe/H] $<-$2.2), exhibits both retrograde and prograde rotation, with an asymmetric $L_z$ distribution towards high retrograde motions, and larger ${\theta }_{\mathrm{orb}}$ than those possessed by the GE dominated inner halo; (e) the Sgr dSph galaxy could induce a long-range dynamical effect on local halo stars. Implication for the formation of the stellar halo are also discussed.

35 pages, 20 figures, some figures were misplaced in the previous version, now fixed

References in corpus (39)

• The 2.5 m Telescope of the Sloan Digital Sky Survey
• Cold streams in early massive hot haloes as the main mode of galaxy formation
• Two Stellar Components in the Halo of the Milky Way
• The SEGUE Stellar Parameter Pipeline. I. Description and Initial Validation Tests
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South
• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• The Dual Origin of Stellar Halos
• The SEGUE Stellar Parameter Pipeline. II. Validation with Galactic Globular and Open Clusters
• Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago
• Kraken reveals itself -- the merger history of the Milky Way reconstructed with the E-MOSAICS simulations
• The stellar halo of the Galaxy
• The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars
• Kinematics of the stellar halo and the mass distribution of the Milky Way using BHB stars
• Reverse engineering the Milky Way
• Prime Focus Spectrograph (PFS) for the Subaru Telescope: Overview, recent progress, and future perspectives
• Disentangling the Galactic Halo with APOGEE: I. Chemical and Kinematical Investigation of Distinct Metal-Poor Populations
• New H-band Stellar Spectral Libraries for the SDSS-III/APOGEE survey
• A Cold, Massive, Rotating Disk Galaxy 1.5 Billion Years after the Big Bang
• A box full of chocolates: The rich structure of the nearby stellar halo revealed by Gaia and RAVE
• Halo Substructure in the SDSS-Gaia Catalogue : Streams and Clumps
• The Milky Way's Disk of Classical Satellite Galaxies in Light of Gaia DR2
• Timing the Early Assembly of the Milky Way with the H3 Survey
• Origin of the excess of high-energy retrograde stars in the Galactic halo
• Kinematic Decoupling of Globular Clusters with Extended Horizontal-Branch
• Evidence for Distinct Components of the Galactic Stellar Halo from 838 RR Lyrae Stars Discovered in the LONEOS-I Survey
• The Pristine survey X: a large population of low-metallicity stars permeates the Galactic disk
• Metallicity and $\alpha$-element Abundance Gradients along the Sagittarius Stream as Seen by APOGEE
• A tale of two populations: surviving and destroyed dwarf galaxies and the build up of the Milky Way's stellar halo
• Quantifying the Stellar Halo's Response to the LMC's Infall with Spherical Harmonics
• Identifying Contributions to the Stellar Halo from Accreted, Kicked-Out, and In Situ Populations
• [CII] 158 Micron Emission from z~4 HI Absorption-Selected Galaxies
• Kinematic Structure in the Galactic Halo at the North Galactic Pole: RR Lyrae and BHB Stars show different kinematics
• The age structure of the Milky Way's halo
• Long-term orbital evolution of Galactic satellites and the effects on their star formation histories
• Constraints on the Galactic Inner Halo Assembly History from the Age Gradient of Blue Horizontal-branch Stars
• Explaining the chemical trajectories of accreted and in-situ halo stars of the Milky Way
• Insights into the Formation and Evolution History of the Galactic Disk System
• Proper motion measurements for stars up to $100$ kpc with Subaru HSC and SDSS Stripe 82

Cited by in corpus (18)

• Reconstructing the Last Major Merger of the Milky Way with the H3 Survey
• The GALAH Survey: Chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and $Gaia$ eDR3
• Action-based distribution function modelling for constraining the shape of the Galactic dark matter halo
• Origin of highly $r$-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy
• Contribution of Gaia Sausage to the Galactic Stellar Halo Revealed by K Giants and Blue Horizontal Branch Stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, Sloan Digital Sky Survey, and Gaia
• A Blueprint for the Milky Way's Stellar Populations. III. Spatial Distributions and Population Fractions of Local Halo Stars
• Dynamics of Stellar Disk Tilting from Satellite Mergers
• Understanding the early stages of galaxy formation using very metal-poor stars from the Hamburg/ESO survey
• Exploring the chemodynamics of metal-poor stellar populations
• Robust Clustering of the Local Milky Way Stellar Kinematic Substructures with Gaia eDR3
• Influence of the Gaia-Sausage-Enceladus on the density shape of the Galactic stellar halo revealed by halo K giants from the LAMOST survey
• Element abundance analysis of the metal-rich stellar halo and high-velocitythick disk in the galaxy
• L-GALAXIES 2020: The formation and chemical evolution of stellar haloes in Milky Way Analogues and galaxy clusters
• The global structure of the Milky Way's stellar halo based on the orbits of local metal-poor stars
• AuriDESI: Mock Catalogues for the DESI Milky Way Survey
• Revealing the Milky Way's Most Recent Major Merger with a Gaia EDR3 Catalog of Machine-Learned Line-of-Sight Velocities
• Search for the metal-weak thick disk from the LAMOST DR5
• Unravelling the role of merger histories in the population of In situ stars: linking IllustrisTNG cosmological simulation to H3 survey