The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar

Abstract

Studying the composition and origin of the inner region of our Galaxy -- the "Galactic bulge" -- is crucial for understanding the formation and evolution of the Milky Way and other galaxies. We present new observational constraints based on a sample of around 18,000 stars in the inner Galaxy, combining Gaia DR3 RVS and APOGEE DR17 spectroscopy. Gaia-RVS complements APOGEE by improving sampling of the metallicity, [Fe/H] in the -2.0 to -0.5 dex range. This work marks the first application of Gaia-RVS spectroscopy to the bulge region, enabled by a novel machine learning approach (hybrid-CNN) that derives stellar parameters from intermediate-resolution spectra with precision comparable to APOGEE's infrared data. We performed full orbit integrations using a barred Galactic potential and applied orbital frequency analysis to disentangle the stellar populations in the inner Milky Way. For the first time, traced by the field stars, we are able to robustly identify the long-sought pressure supported bulge. We show this stellar population to be chemically and kinematically distinct from the other main components co-existing in the same region. The spheroidal bulge has a metallicity distribution function (MDF) peak at around -0.70 dex extending to solar value, is dominated by a high-[alpha/Fe] population with almost no dependency on metallicity, consistent with very rapid early formation, predating the thick disc and the bar. We find evidence that the bar has influenced the dynamics of the spheroidal bulge, introducing a mild triaxiality and radial extension. We identify a group of stars on X4 orbits, likely native to the early spheroid, as this population mimics the chemistry of the spheroidal bulge, with a minor contamination from the more metal-poor ([Fe/H] < -1.0) halo. We find the inner-thick disc to be kinematically hotter (mean Vphi ~125 km/s) than the local-thick disc. ...

arXiv commentAccepted for publication in A&A on 25-Nov-2025. Abstract abridged

References in corpus (102)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Gaia Data Release 3: Summary of the content and survey properties
• Local Kinematics and the Local Standard of Rest
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• The Galaxy in Context: Structural, Kinematic and Integrated Properties
• galpy: A Python Library for Galactic Dynamics
• The merger that led to the formation of the Milky Way's inner stellar halo and thick disk
• Exploring the Milky Way stellar disk. A detailed elemental abundance study of 714 F and G dwarf stars in the Solar neighbourhood
• The mass distribution and gravitational potential of the Milky Way
• Co-formation of the Galactic disc and the stellar halo
• VISTA Variables in the Via Lactea (VVV): The public ESO near-IR variability survey of the Milky Way
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• Gaia Early Data Release 3: Photometric content and validation
• What determines the strength and the slowdown rate of bars ?
• Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics
• AGAMA: Action-based galaxy modelling architecture
• Asteroseismology of Solar-Type and Red-Giant Stars
• Dynamics of Barred Galaxies
• Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo
• Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. V. Evidence for a wide age distribution and a complex MDF
• Origin of the system of globular clusters in the Milky Way
• Dynamical modelling of the Galactic bulge and bar: the Milky Way's bar pattern speed, stellar, and dark matter mass distribution
• Secular Evolution in Disk Galaxies
• The formation and assembly history of the Milky Way revealed by its globular cluster population
• The Metal Content of Bulge Field Stars from FLAMES-GIRAFFE Spectra. I. Stellar parameters and Iron Abundances
• Bulge Formation by the Coalescence of Giant Clumps in Primordial Disk Galaxies
• ARGOS III: Stellar Populations in the Galactic Bulge of the Milky Way
• Dynamical Friction and the Distribution of Dark Matter in Barred Galaxies
• Gaia Data Release 3: Analysis of RVS spectra using the General Stellar Parametriser from spectroscopy
• Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history
• The Bulge Radial Velocity Assay (BRAVA): II. Complete Sample and Data Release
• The metallicity distribution of bulge clump giants in Baade's Window
• Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia EDR3 stars brighter than G=18.5
• StarHorse: A Bayesian tool for determining stellar masses, ages, distances, and extinctions for field stars
• Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy
• Gaia Data Release 3: Astrophysical parameters inference system (Apsis) I -- methods and content overview
• Gaia Data Release 3: Processing and validation of BP/RP low-resolution spectral data
• Chemodynamical history of the Galactic Bulge
• From dawn till disk: Milky Way's turbulent youth revealed by the APOGEE+Gaia data
• Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants
• Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. VI. Age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way
• Abundances of Baade's Window Giants from Keck/HIRES Spectra: I. Stellar Parameters and [Fe/H] Values
• Age dissection of the Milky Way discs: red giants in the Kepler field
• 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
• Deciphering the 3D structure of the old Galactic bulge from the OGLE RR Lyrae stars
• The Sixth Data Release of the Radial Velocity Experiment (RAVE) -- II: Stellar Atmospheric Parameters, Chemical Abundances and Distances
• Reverse engineering the Milky Way
• Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way's quiescent merger history and in-situ bulge
• The Gaia-ESO Survey: metallicity and kinematic trends in the Milky Way bulge
• The chemical characterisation of halo substructure in the Milky Way based on APOGEE
• The GIRAFFE Inner Bulge Survey (GIBS). I. Survey Description and a kinematical map of the Milky Way bulge
• The Gaia-ESO Public Spectroscopic Survey: Motivation, implementation, GIRAFFE data processing, analysis, and final data products
• The Poor Old Heart of the Milky Way
• The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3
• StarHorse results for spectroscopic surveys + Gaia DR3: Chrono-chemical populations in the solar vicinity, the genuine thick disk, and young-alpha rich stars
• PLATO as it is: a legacy mission for Galactic archaeology
• The First Detailed Abundances for M giants in the inner Bulge from Infrared Spectroscopy
• Selecting accreted populations: metallicity, elemental abundances, and ages of the Gaia-Sausage-Enceladus and Sequoia populations
• The Orbital Evolution Induced by Baryonic Condensation in Triaxial Halos
• Alpha Enhancement and the Metallicity Distribution Function of Plaut's Window
• The GALAH Survey: Chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and $Gaia$ eDR3
• Peanuts, brezels and bananas: food for thought on the orbital structure of the Galactic bulge
• Reconstructing the Disrupted Dwarf Galaxy $Gaia$-Sausage/Enceladus Using its Stars and Globular Clusters
• The disc origin of the Milky Way bulge: Dissecting the chemo-morphological relations using N-body simulations and APOGEE
• Detailed abundances for M giants in two inner bulge fields from Infrared Spectroscopy
• The Pristine Inner Galaxy Survey (PIGS) I: Tracing the kinematics of metal-poor stars in the Galactic bulge
• How Many Components? Quantifying the Complexity of the Metallicity Distribution in the Milky Way Bulge with APOGEE
• A Vertical Resonance Heating Model for X- or Peanut-Shaped Galactic Bulges
• Three Mechanisms for Bar Thickening
• The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis
• A unified framework for the orbital structure of bars and triaxial ellipsoids
• Growth of a peanut shaped bulge via resonant trapping of stellar orbits in the vertical Inner Lindblad Resonances
• The relation between chemical abundances and kinematics of the Galactic disc with RAVE
• The Bulge Radial Velocity Assay for RR Lyrae stars (BRAVA-RR) DR2: a Bimodal Bulge?
• The Pristine Inner Galaxy Survey (PIGS) II: Uncovering the most metal-poor populations in the inner Milky Way
• Metallicity-dependendent kinematics and morphology of the Milky Way bulge
• A new code for orbit analysis and Schwarzschild modelling of triaxial stellar systems
• The origin of stellar populations in the Galactic bulge from chemical abundances
• The Bulge Metallicity Distribution from the APOGEE Survey
• On the orbits that generate the X-shape in the Milky Way bulge
• The Stellar Halo in the Inner Milky Way: Predicted Shape and Kinematics
• Box/peanut-shaped bulges in action space
• Discovery of the local counterpart of disc galaxies at z > 4: The oldest thin disc of the Milky Way using Gaia-RVS
• Orbital ingredients for cooking X-structures in edge-on galaxies
• Weighing the two stellar components of the Galactic Bulge
• The orbital content of bars: The origin of 'non-x1-tree', bar-supporting orbits
• Orbital support and evolution of flat profiles of bars (shoulders)
• The Galactic Stellar Disc
• A census of new globular clusters in the Galactic bulge
• Building CX peanut-shaped disk galaxy profiles. The relative importance of the 3D families of periodic orbits bifurcating at the vertical 2:1 resonance
• The imprint of clump formation at high redshift. II. The chemistry of the bulge
• Abundance analysis of APOGEE spectra for 58 metal-poor stars from the bulge spheroid
• PANORAMIC: Discovery of an Ultra-Massive Grand-Design Spiral Galaxy at $z\sim 5.2$
• Blanco DECam Bulge Survey (BDBS) III: A new view of the double red clump in the Milky Way bulge through luminosity and color distribution
• Chrono-chemodynamical analysis of the globular cluster NGC 6355: Looking for the fundamental bricks of the Bulge
• Cluster Ages to Reconstruct the Milky Way Assembly (CARMA). II. The age-metallicity relation of Gaia-Sausage-Enceladus globular clusters
• Frequencies, chaos and resonances: a study of orbital parameters of nearby thick disc and halo stars
• Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal) -- III. Age and metallicity distribution of Gaia-Sausage-Enceladus stars near the Sun
• Revisited parameters for the twin bulge globular clusters NGC 6528 and NGC 6553