The Structure, Populations and Kinematics of the Milky Way central and inner Bulge with OGLE, APOGEE and Gaia data

TL;DR

This study combines OGLE, APOGEE, and Gaia data to analyze the structure, kinematics, and chemical properties of the Milky Way bulge, revealing multiple stellar populations, their orbital behaviors, and supporting a pseudo-bulge origin with a preference for a boxy shape.

Contribution

It provides a detailed chemo-dynamical analysis of the Milky Way bulge using combined datasets, identifying distinct populations and supporting the pseudo-bulge formation scenario.

Findings

Inner bulge RR Lyrae stars align with the Galactic bar.

Kinematic differences distinguish bulge populations.

Data favor a boxy bulge model over X-shaped.

Abstract

We present an analysis of the structure, kinematics, and chemo-dynamical properties of the Milky Way bulge using RR Lyrae stars from OGLE, and giant stars from APOGEE and Gaia that have distances placing them in the inner Galaxy. Firstly, using a sample of 1,879 ab-type RR Lyrae stars (RRabs) from OGLE-IV, we identified three populations: central bulge RRabs, the inner bulge RRabs, and halo or disk interlopers, based on their apocenters derived from orbital integration. Inner bulge RRabs kinematically align with the Galactic bar, while central bulge RRabs show slower rotation with lower velocity dispersion. Higher velocity dispersion stars were identified as halo/disk interlopers. Then, orbital analysis of 28,188 APOGEE Red Clump and Red Giant Branch stars revealed kinematic properties consistent with RRabs, and the chemical abundance distribution displayed a bimodal stellar density pattern, suggesting complex star evolution histories and slightly different star formation histories for the inner bulge and central bulge. The differences in the density distribution on the $|\mathrm{Z}|_{\text{max}}$-eccentricity plane for the central bulge, inner bulge, and interlopers are clearly detected. It is found that the classification of bulge stars based on orbital parameters, rather than solely on metallicity, provides a more accurate population separation. As the inner bulge, which contains the highest fraction of stars, traces the bar formed by the instability of the Galactic disk, our results support that pseudo-bulge is the primary origin of the bulge. Furthermore, fitting the observed data to both the boxy and X-shaped bulge models indicated a preference for the boxy bulge.