The far side of the Galactic bar/bulge revealed through semi-regular variables

TL;DR

This study introduces a new method to measure distances to luminous red giants in the Galactic bulge using OGLE data, revealing the structure and kinematics of the Milky Way's bar and bulge beyond the Galactic center with implications for understanding its dynamics.

Contribution

The paper presents a novel distance measurement technique anchored to the LMC, applied to OGLE data, and provides the first constraints on the Milky Way's velocity field beyond the Galactic center.

Findings

The Galactic center distance is measured as approximately 8.11 kpc.

The bar is bi-symmetric and aligned with the inner disk.

No major vertical structure is observed in the bulge region.

Abstract

The Galactic bulge and bar are critical to our understanding of the Milky Way. However, due to the lack of reliable stellar distances, the structure and kinematics of the bulge/bar beyond the Galactic center have remained largely unexplored. Here, we present a method to measure distances of luminous red giants using a period-amplitude-luminosity relation anchored to the Large Magellanic Cloud, with random uncertainties of 10-15% and systematic errors below 1-2%. We apply this method to data from the Optical Gravitational Lensing Experiment (OGLE) to measure distances to $190,302$ stars in the Galactic bulge and beyond out to 20 kpc. Using this sample we measure a distance to the Galactic center of $R_0$ = $8108\pm106_{\rm stat}\pm93_{\rm sys}$ pc, consistent with astrometric monitoring of stars orbiting Sgr A*. We cross-match our distance catalog with Gaia DR3 and use the subset of $39,566$ overlapping stars to provide the first constraints on the Milky Way's velocity field ($V_R,V_\phi,V_z$) beyond the Galactic center. We show that the $V_R$ quadrupole from the bar's near side is reflected with respect to the Galactic center, indicating that the bar is both bi-symmetric and aligned with the inner disk, and therefore dynamically settled along its full extent. We also find that the vertical height $V_Z$ map has no major structure in the region of the Galactic bulge, which is inconsistent with a current episode of bar buckling. Finally, we demonstrate with N-body simulations that distance uncertainty plays a major factor in the alignment of the major and kinematic axes of the bar and distribution of velocities, necessitating caution when interpreting results for distant stars.