Towards Unveiling the Origins of the Milky Way Bulge through Multi-band-Messenger Sky Surveys

Abstract

We analyze the structure and chemo-dynamical properties of the Galactic bulge using ab-type RR Lyrae stars (RRabs) from OGLE-IV and giant stars from APOGEE and Gaia. Orbital integration of 1,879 RRab variables reveals three populations: central bulge, inner bulge, and halo/disk contaminants. Inner bulge RRabs display bar-like kinematics, whereas central bulge stars show slower rotation and lower dispersion. APOGEE data for 28,188 stars confirm these kinematic trends and reveal a bimodal chemical distribution, indicating distinct formation pathways. Our results support a pseudo-bulge origin of the inner bulge through disk instability, with the overall morphology better described as boxy rather than X-shaped. Through the integration of multi-messenger, multi-band data, our collaboration aims to provide deeper insights into the physical properties and evolutionary history of the Galactic bulge.

Figures (5)

• Figure 1: Left: Galactic positions of 1,879 RRab stars with orbits (yellow) and 28,188 APOGEE stars (gray). Right: Heliocentric distance histograms for the OGLE and APOGEE samples: 17,817 RRab stars before orbital selection (blue), 1,879 RRab stars after orbital selection (orange), and 28,188 APOGEE stars (green).
• Figure 2: Top: The mean LOS velocity and velocity dispersion maps of different stellar populations as a function of Galactic longitude. Blue points represent central bulge RRabs ($r_{\text{apo}} < 1.8 \, \text{kpc}$), orange points represent inner bulge RRabs ($1.8 \, \text{kpc} \leq r_{\text{apo}} < 3.5 \, \text{kpc}$), and green points represent halo interlopers ($r_{\text{apo}} \geq 3.5 \, \text{kpc}$). Bottom: The mean $v^{*}_{l}$ and velocity dispersion maps for the same stars.
• Figure 3: Top: The mean LOS velocity and velocity dispersion maps of different stellar populations as a function of Galactic longitude. Blue points represent central bulge RGBs and RCs ($r_{\text{apo}} < 1.8 \, \text{kpc}$), orange points represent inner bulge RGBs and RCs ($1.8 \, \text{kpc} \leq r_{\text{apo}} < 3.5 \, \text{kpc}$), and green points represent halo/disk interlopers ($r_{\text{apo}} \geq 3.5 \, \text{kpc}$). Bottom: The mean $v^{*}_{l}$ and velocity dispersion maps for the same stars.
• Figure 4: Two-dimensional chemical abundance plots of APOGEE RGBs and RCs in the central bulge (left), inner bulge (middle), and halo/disk interlopers (right). From top to bottom, respectively, are [$\alpha$/Fe] vs. [Fe/H], [Mg/Fe] vs. [Fe/H], [O/Fe] vs. [Fe/H], [Mn/O] vs. [O/H], and [C/N] vs. [Fe/H]. The [Mn/O] patterns suggest different star formation histories for the inner bulge and central bar.
• Figure 5: Distributions of stars in the [$\mathrm{Fe/H}$]–[$\alpha/\mathrm{Fe}$] plane, color-coded by their cylindrical velocity components: radial velocity ($\mathrm{V_R}$, left column), azimuthal velocity ($\mathrm{V_\phi}$, middle column), and vertical velocity ($\mathrm{V_Z}$, right column). The top row corresponds to stars located in the central bulge region, while the bottom row shows stars in the inner bulge.