Kinematic, Elemental and Structural Dependences on Metallicity in the Galactic Bulge

TL;DR

This study investigates how metallicity influences the kinematics, chemistry, and structure of Galactic Bulge stars by combining APOGEE DR17 abundances with Gaia DR3 astrometry. It derives orbital actions in an axisymmetric potential and identifies six MDF peaks using both find_peaks and Gaussian Mixture Models, revealing a chemically diverse Bulge with metal-rich and metal-poor subpopulations. Metal-poor stars exhibit larger action dispersions and more irregular kinematics, while metal-rich stars are kinematically coherent and likely linked to disk origins, with retrograde stars confined near the center and decreasing in frequency at higher [Fe/H]. In chemical space, the Bulge shows bimodality in multiple elements with a knee near [Fe/H] ≈ $-0.2$, and the $[Mg/Mn]-[Al/Fe]$ plane suggests a predominantly in-situ or disk-migrated origin for many metal-rich stars, including a thick-disk-like component among low-Al metal-rich stars. Structurally, the Bulge is best described by a boxy profile, though metal-rich stars show increasing evidence for an X-shaped component, indicating a nuanced interplay between secular bar dynamics and possible merger effects.

Abstract

We selected Bulge stars from APOGEE DR17 cross-matched with astrometric data from \textit{Gaia} DR3. Bulge stars were divided into sub-samples with line-of-sight velocity dispersion analyzed and the peaks of MDF were detected by both Gaussian Mixture Models (GMM) and \texttt{scipy.signal.find\_peaks}. GMM is also conducted to kinematically distinguish the metal-poor and metal-rich populations. Analyses were put on 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/boxy) of Bulge stars with different metallicities were analyzed through least-squares fitting based on the analytical Bulge models. 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 sub-samples, while absent in metal-poor sub-samples. 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 $\sim1$ 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 from 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.

Figures (10)

• Figure 1: Left panel: line-of-sight velocity dispersions (in units of km/s) across $0<|b|<10^\circ$ along the minor axis ($|l|<2^\circ$) of the Bulge. The solid red and blue lines represent metal-rich and metal-poor stars, respectively. The dashed lines represent the metal-rich and metal-poor sub-samples, while the black cross indicates EMR stars. Right panel: the MDF of the Bulge. Six peaks have been identified with a prominence of 0.25, as indicated by the black arrows.
• Figure 2: The kinematic dependence on metallicity in the Galactic Bulge. The red dot and error bar indicate the median values, as well as the 16th and 84th percentiles for each equal-width [Fe/H] bin. Top row: the distributions in action-metallicity plane. Note that the median values of $J_{\phi}$ in each bin could be well fitted by a quadratic function. Bottom row: the distributions in $e$/$Z_{\text{max}}$/$\eta$ planes.
• Figure 3: The dependences of the action dispersions (defined as standard deviations) and orbital dispersions on metallicity is examined. The action dispersions of metal-poor stars are larger than those of metal-rich stars, suggesting that metal-poor stars may have multiple dynamical origins. The error bar indicates the standard error, which is given by $\sigma/\sqrt{N-1}$.
• Figure 4: The heatmap illustrates the similarity of the three clusters based on the distributions of $J_r$, $J_z$, and [Fe/H]. The rightmost panel shows the distributions of the three clusters in $J_r / J_{\text{total}}$-$|J_{\phi}| / J_{\text{total}}$ plane, with the median, 16th percentile, and 84th percentile of [Fe/H] for each cluster indicated in the legend. Cluster 0 has a wide span in the plane and could potentially be clustered into smaller groups when additional quantities are considered. Cluster 1 is the most metal-poor and is supported by both radial and rotational motion. Cluster 2 is the least metal-poor and is mainly supported by rotational motion.
• Figure 5: The histograms of retrograde and prograde stars are presented in terms of elemental abundances and orbital parameters. The retrograde stars show a metal-poor peak at $\sim-0.6$ and are more metal-poor than prograde stars. The retrograde stars are also slightly [C/N] and $\alpha$ enhanced than prograde stars, indicating that they are relatively older. The retrograde stars are mainly confined within $r_{\text{GC}}\sim2$ kpc while the prograde stars have a large $r_{\text{GC}}$ more than 3 kpc. Both retrograde and prograde stars have $Z_{\text{max}}<3$ kpc.