Dynamical properties of high-[Mg/Fe] stars in the Milky Way bar region

Abstract

The origin of the high-alpha component of the Galactic bulge remains debated, unlike the bar-driven origin of the low-alpha bulge. We examine the metallicity-dependent dynamical properties of high-[Mg/Fe] stars in the bar region, using samples of low- and high-[Mg/Fe] stars from APOGEE DR17, complemented by the PIGS catalogue of [Fe/H] $<-1$ stars. The mean Galactocentric rotational velocity ${\overline{V}}_{\varphi }(R)$ is nearly cylindrical for both low- and high-[Mg/Fe] stars across the bulge and outer bar. ${\overline{V}}_{\varphi }(R)$ of high-[Mg/Fe] stars with [Fe/H] $\ge -0.6$ is similar within errors to low-[Mg/Fe] stars in the bulge, and $10-20\%$ lower in the outer bar. The mean radial velocity field of these stars exhibits a quadrupole pattern similar to low-[Mg/Fe] stars. Orbit integrations in realistic barred Galactic potentials show that these model-independent properties correspond to a peanut bulge in the orbital density distributions for high-[Mg/Fe] stars with [Fe/H] $\ge -0.6$, transitioning toward a more spheroidal structure at lower metallicities. Additionally, ${\overline{V}}_{\varphi }$ ([Fe/H]) increases steeply as metallicity rises from about [Fe/H] $\sim -1.3$, resembling the spin-up observed at larger Galactic radii. This is accompanied by a transition in the dominant orbit families, from co- and counter-rotating cloud A and ${\mathrm{x}}_4$ orbits at low metallicities to co-rotating bar-supporting ${\mathrm{x}}_1$ family tree, box, and cloud A orbits at solar metallicity. Our results strengthen the case that the bulk of the high-[Mg/Fe] component in the bar region evolved from an alpha-enhanced disc, while metal-poor stars with [Fe/H] $<-1$ trace a more turbulent origin.

arXiv comment18 pages, 16 PDF figures; accepted in MNRAS; originally submitted to MNRAS in February 2025

References in corpus (83)

• Local Kinematics and the Local Standard of Rest
• The Galaxy in Context: Structural, Kinematic and Integrated Properties
• The Chemical Evolution of the Galaxy: the two-infall model
• AGAMA: Action-based galaxy modelling architecture
• The Secular Evolution of Disk Structural Parameters
• Vertical waves in the solar neighbourhood in Gaia DR2
• Mapping the Three-Dimensional Density of the Galactic Bulge with VVV Red Clump Stars
• The Emergence of the Thick Disk in a CDM Universe
• Target Selection for the Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• Dynamical modelling of the Galactic bulge and bar: the Milky Way's bar pattern speed, stellar, and dark matter mass distribution
• The Structure of the Milky Way's Bar Outside the Bulge
• Our Milky Way as a Pure-Disk Galaxy -- A Challenge for Galaxy Formation
• 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
• ARGOS III: Stellar Populations in the Galactic Bulge of the Milky Way
• Two Red Clumps and the X-Shaped Milky Way Bulge
• Target Selection for the SDSS-IV APOGEE-2 Survey
• APOGEE Data Releases 13 and 14: Data and Analysis
• The Pristine survey I: Mining the Galaxy for the most metal-poor stars
• The Split Red Clump of the Galactic Bulge from OGLE-III
• The APOGEE red-clump catalog: Precise distances, velocities, and high-resolution elemental abundances over a large area of the Milky Way's disk
• StarHorse: A Bayesian tool for determining stellar masses, ages, distances, and extinctions for field stars
• Life in the fast lane: a direct view of the dynamics, formation, and evolution of the Milky Way's bar
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• ARGOS IV: The Kinematics of the Milky Way Bulge
• Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants
• Galactic Archaeology with asteroseismic ages: evidence for delayed gas infall in the formation of the Milky Way disc
• Improved GRAVITY astrometric accuracy from modeling of optical aberrations
• The origin of the split red clump in the Galactic bulge of the Milky Way
• From the bulge to the outer disc: StarHorse stellar parameters, distances, and extinctions for stars in APOGEE DR16 and other spectroscopic surveys
• Separation of Stellar Populations by an Evolving Bar: Implications for the Bulge of the Milky Way
• Simultaneous calibration of spectro-photometric distances and the Gaia DR2 parallax zero-point offset with deep learning
• Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way's quiescent merger history and in-situ bulge
• Gaia Data Release 3: Mapping the asymmetric disc of the Milky Way
• The Proper Motion of Sagittarius A*: III. The Case for a Supermassive Black Hole
• The GIRAFFE Inner Bulge Survey (GIBS). I. Survey Description and a kinematical map of the Milky Way bulge
• Origin of chemically distinct discs in the Auriga cosmological simulations
• NMAGIC: Fast Parallel Implementation of a Chi-Squared-Made-to-Measure Algorithm for Modelling Observational Data
• Sub-Galactic Clumps at High Redshift: A Fragmentation Origin?
• The Gaia-ESO Survey: Exploring the complex nature and origins of the Galactic bulge populations
• Kinematics at the Edge of the Galactic Bulge: Evidence for Cylindrical Rotation
• Unifying boxy bulge and planar long bar in the Milky Way
• The Poor Old Heart of the Milky Way
• The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3
• Chemical Evolution of the Galactic Bulge as Derived from High-Resolution Infrared Spectroscopy of K and M Red Giants
• Internal calibration of Gaia BP/RP low-resolution spectra
• Spectro-photometric distances to stars: a general-purpose Bayesian approach
• The Gravitational Force Field of the Galaxy Measured From the Kinematics of RR Lyrae in Gaia
• Final Targeting Strategy for the SDSS-IV APOGEE-2S Survey
• Final Targeting Strategy for the SDSS-IV APOGEE-2N Survey
• Fluctuations in galactic bar parameters due to bar-spiral interaction
• APOGEE DR16: a multi-zone chemical evolution model for the Galactic disc based on MCMC methods
• Two Disk Components from a Gas Rich Disk-Disk Merger
• The disc origin of the Milky Way bulge: Dissecting the chemo-morphological relations using N-body simulations and APOGEE
• How Many Components? Quantifying the Complexity of the Metallicity Distribution in the Milky Way Bulge with APOGEE
• Why the Milky Way's bulge is not only a bar formed from a cold thin disk
• APOGEE Kinematics I: Overview of the Kinematics of the Galactic Bulge as Mapped by APOGEE
• APOGEE DR14/DR15 Abundances in the Inner Milky Way
• A Tale of Two Disks: Mapping the Milky Way with the Final Data Release of APOGEE
• A direct measurement of the distance to the Galactic center using the kinematics of bar stars
• Gas Dynamics in the Galaxy: Total Mass Distribution and the Bar Pattern Speed
• A unified framework for the orbital structure of bars and triaxial ellipsoids
• The disc origin of the Milky Way bulge
• The Bulge Radial Velocity Assay for RR Lyrae stars (BRAVA-RR) DR2: a Bimodal Bulge?
• The stellar mass distribution of the Milky Way's bar: an analytic model
• The Pristine Inner Galaxy Survey (PIGS) II: Uncovering the most metal-poor populations in the inner Milky Way
• Secular evolution and cylindrical rotation in boxy/peanut bulges: impact of initially rotating classical bulges
• 3D kinematics through the X-shaped Milky Way bulge
• Formation, vertex deviation and age of the Milky Way's bulge: input from a cosmological simulation with a late-forming bar
• The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2
• The bimodal [Mg/Fe] versus [Fe/H] bulge sequence as revealed by APOGEE DR14
• Disk settling and dynamical heating: histories of Milky Way-mass stellar disks across cosmic time in the FIRE simulations
• Spin-up of low mass classical bulges in barred galaxies
• The Stellar Halo in the Inner Milky Way: Predicted Shape and Kinematics
• The Three-Phase Evolution of the Milky Way
• The analysis of realistic Stellar Gaia mock catalogues. I. Red Clump Stars as tracers of the central bar
• Birth of the Galactic Disk Revealed by the H3 Survey
• The Milky Way's bulge star formation history as constrained from its bimodal chemical abundance distribution
• Spin-up of massive classical bulges during secular evolution
• A2A: 21,000 bulge stars from the ARGOS survey with stellar parameters on the APOGEE scale
• Kinematics of the X-shaped Milky Way Bulge: Expectations from a Self-consistent N-body Model
• Orbit-Superposition Dynamical Modeling of Barred Galaxies
• The imprint of clump formation at high redshift. II. The chemistry of the bulge
• Rediscovering the Milky Way with orbit superposition approach and APOGEE data I. Method validation