Kinematic, elemental, and structural dependences on metallicity in the Galactic bulge

School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR China

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Received: 16 June 2025
Accepted: 22 October 2025

Abstract

Context. The Galactic bulge is known to have multiple stellar populations according to the metallicity-distribution-function (MDF) peaks. These populations show varying properties of kinematic, elemental, and spatial distributions.

Aims. This study aims to investigate metallicity-dependent variations in the kinematic properties, elemental abundances, and spatial distributions of Galactic-bulge stars and distinguishes these populations.

Methods. We selected bulge stars from APOGEE DR17 cross-matched with astrometric data from Gaia DR3. Bulge stars were divided into subsamples with the line-of-sight velocity dispersion analyzed, and the peaks of MDF were detected by both Gaussian mixture models (GMMs) and scipy.signal.find_peaks. Orbital parameters (e.g., actions) were calculated via orbital integration, and the kinematic dependence on metallicity was investigated. A GMM was also conducted to kinematically distinguish the metal-poor and metal-rich populations. Analyses were carried out of the bulge stars (including retrograde stars), their elemental abundances, and the [Mg/Mn]–[Al/Fe] plane to investigate potential accreted components. Finally, the shapes (X-shaped or boxy) of bulge stars with different metallicities were analyzed through least-squares fitting based on the analytical bulge models.

Results. By studying the kinematic, elemental, and structural dependences on metallicity for bulge stars, our findings are concluded as follows. (1) Six peaks are detected in the bulge MDF, encompassing values reported in previous studies, suggesting a complex composition of bulge populations. (2) An inversion relationship is well observed in metal-rich subsamples, while absent in metal-poor subsamples. (3) Metal-poor populations exhibit larger dispersions than metal-rich stars (which is also revealed by GMM decomposition), suggesting that metal-rich stars are kinematically coherent. (4) Retrograde stars are confined to ∼1 kpc of the Galactic center, with their relative fraction decreasing at higher [Fe/H] – a trend potentially linked to the “spin-up” process of Galactic disks. (5) Metal-rich bulge stars with [Al/Fe] < −0.15 are likely associated with a disk-accreted substructure, while all elemental planes exhibit bimodality, but Na abundances rise monotonically with metallicity. (6) In general, stars with all metallicities support a boxy profile.