The Metallicity Distribution of the Milky Way Bulge

Abstract

The Galactic bulge of the Milky Way is made up of stars with a broad range of metallicity, -3.0 < [Fe/H] < 1 dex. The mean of the Metallicity Distribution Function (MDF) decreases as a function of height z from the plane and, more weakly, with galactic radius. The most metal rich stars in the inner Galaxy are concentrated to the plane and the more metal poor stars are found predominantly further from the plane, with an overall vertical gradient in the mean of the MDF of about -0.45 dex/kpc. This vertical gradient is believed to reflect the changing contribution with height of different populations in the inner-most region of the Galaxy. The more metal rich stars of the bulge are part of the boxy/peanut structure and comprise stars in orbits which trace out the underlying X-shape. There is still a lack of consensus on the origin of the metal poor stars ([Fe/H] < -0.5) in the region of the bulge. Some studies attribute the more metal poor stars of the bulge to the thick disk and stellar halo that are present in the inner region, and other studies propose that the metal poor stars are a distinct "old spheroid" bulge population. Understanding the origin of the populations that make up the MDF of the bulge, and identifying if there is a unique bulge population which has formed separately from the disk and halo, has important consequences for identifying the relevant processes in the the formation and evolution of the Milky Way.

Accepted for publication in PASA as an invited review as part of a special issue on the Galactic Bulge

References in corpus (9)

• Chemical Cartography with APOGEE: Metallicity Distribution Functions and the Chemical Structure of the Milky Way Disk
• The Cannon: A data-driven approach to stellar label determination
• The Structure of the Milky Way's Bar Outside the Bulge
• Tracing chemical evolution over the extent of the Milky Way's Disk with APOGEE Red Clump Stars
• Made-to-Measure models of the Galactic Box/Peanut bulge: stellar and total mass in the bulge region
• The First Detailed Abundances for M giants in the inner Bulge from Infrared Spectroscopy
• The Gaia-ESO Survey: the most metal-poor stars in the Galactic bulge
• Chemistry of the Most Metal-poor Stars in the Bulge and the z > 10 Universe
• A high-velocity bulge RR Lyrae variable on a halo-like orbit

Cited by in corpus (32)

• The Close Binary Fraction of Solar-type Stars is Strongly Anti-correlated with Metallicity
• From Nuclei to the Cosmos: Tracing Heavy-Element Production with the Oldest Stars
• The GIRAFFE Inner Bulge Survey (GIBS) III. Metallicity distributions and kinematics of 26 Galactic bulge fields
• Chemodynamics of barred galaxies in cosmological simulations: On the Milky Way's quiescent merger history and in-situ bulge
• The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3
• Characterizing the continuous gravitational-wave signal from boson clouds around Galactic isolated black holes
• KMOS view of the Galactic Centre - II. Metallicity distribution of late-type stars
• The disc origin of the Milky Way bulge: Dissecting the chemo-morphological relations using N-body simulations and APOGEE
• APOGEE DR14/DR15 Abundances in the Inner Milky Way
• Galactic Center Gas Clouds and Novel Bounds on Ultra-Light Dark Photon, Vector Portal, Strongly Interacting, Composite, and Super-Heavy Dark Matter
• The Chemical Composition of the Galactic Bulge and Implications for its Evolution
• The Similarity of Abundance Ratio Trends and Nucleosynthetic Patterns in the Milky Way Disk and Bulge
• Radii of 88 M Subdwarfs and Updated Radius Relations for Low-Metallicity M Dwarf Stars
• The COMBS Survey -- II. Distinguishing the Metal-Poor Bulge from the Halo Interlopers
• The COMBS survey I: Chemical Origins of Metal-Poor Stars in the Galactic Bulge
• Kinematics in the Galactic Bulge with APOGEE: II. High-Order Kinematical Moments and Comparison to Extragalactic Bar Diagnostics
• A bottom-heavy initial mass function for the likely-accreted blue-halo stars of the Milky Way
• Detailed abundance analysis of a metal-poor giant in the Galactic Center
• Chemical enrichment of stars due to accretion from ISM during the Galaxy's assembly
• The most metal-rich asymptotic giant branch stars
• Is the molecular KS relationship universal down to low metallicities?
• The VIRUS-P Exploration of Nearby Galaxies (VENGA): The stellar populations and assembly of NGC 2903's bulge, bar, and outer disc
• A machine learning approach to photometric metallicities of giant stars
• Unravelling stellar populations in the Andromeda Galaxy
• New Galactic $\beta$ Lyrae-type Binaries Showing Superorbital Photometric Cycles
• Predicted Trends in Milky Way Bulge Proper Motion Rotation Curves: future Prospects for HST and LSST
• Modelling the chemical evolution of the Milky Way
• Rediscovering the Milky Way with orbit superposition approach and APOGEE data III. Panoramic view of the bulge
• Capivara: A Spectral-based Segmentation Method for IFU Data Cubes
• The COMBS Survey -- III. The Chemodynamical Origins of Metal-Poor Bulge Stars
• White dwarfs in the building blocks of the Galactic spheroid
• The Two-infall Model Revisited: Constraints on Milky Way Bulge Assembly from >30,000 Galactic Chemical Evolution Models and Machine Learning