Kinematic and extinction analysis of a potential spiral arm beyond the Galactic bar

TL;DR

This study investigates a double red clump feature at low Galactic latitudes, providing evidence for a distant spiral arm beyond the Galactic bar through kinematic and extinction analysis.

Contribution

It offers a novel analysis combining proper motion, extinction data, and simulations to identify a potential spiral arm beyond the Galactic bar.

Findings

Two RC features are kinematically distinct with a proper motion difference of -0.16 mas/yr.

Extinction difference between RCs is about 0.05 mag, indicating little interstellar material.

Results support the secondary RC tracing a distant structure, possibly a spiral arm.

Abstract

Determining the structure of the Milky Way is essential for understanding its morphology, dynamics, and evolution. However, studying its innermost regions is challenging due to high extinction and crowding. The detection of a double red clump (RC; core-helium-burning stars) feature at very low Galactic latitudes suggests the presence of a spiral arm beyond the Galactic bar, providing new insights into the Galaxy's structure along this complex line of sight. We evaluate this possibility by analysing the proper motion and extinction distributions of the detected RC features. We constructed proper motion and extinction difference maps to investigate the kinematic and reddening properties of the RC populations, and the kinematic differences were validated using N-body simulations of a Milky Way-like galaxy. We find that the two RC features are kinematically distinct, with a relative proper motion difference of $-0.16\pm0.02\, mas/yr$ in the component parallel to the Galactic plane. This difference can be explained by Galactic rotation if the two RCs lie at different distances, consistent with the simulations. The extinction towards the secondary RC is also $\sim0.05$ mag higher than that of the primary RC. Additionally, we estimate that the extinction difference between the RC features corresponds to only $\sim5\%$ of the total extinction from Earth to the first RC, suggesting little interstellar material between the farthest edge of the Galactic bar and the kinematically distinct structure traced by the secondary RC. Using $JK_s$ photometry, we derive $A_J/A_{K_s}=3.34\pm0.07$, consistent with previous results and showing no significant variation across fields or along the line of sight. The results support the secondary clump tracing a distant structure, possibly a spiral arm, although we cannot exclude that the population belongs to the disc.