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

¹ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
² Max-Planck Institut für Extraterrestrische Physik, 85748 Garching, Germany
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei München, Germany
⁴ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁶ Institute of Space Sciences & Astronomy, University of Malta, Msida MSD 2080, Malta

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 February 2025
Accepted: 8 October 2025

Abstract

Context. 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.

Aims. We aim to evaluate the presence of this spiral arm by analysing the proper motion and extinction distributions of the detected RC features.

Methods. We constructed proper motion and extinction difference maps to investigate the kinematic and reddening properties of the RC populations. We also confirmed the kinematic difference we see in observational data with N-body simulations of a Milky Way-like galaxy.

Results. We find that the two RC features are kinematically distinct, with a relative proper motion difference of −0.16 ± 0.02 mas/yr in the component parallel to the Galactic plane. This difference can be explained by Galactic rotation if the two RC features are located at different distances along the line of sight, consistent with our simulation results. The extinction towards the secondary RC is also ~0.05 mag higher than that of the primary RC. Additionally, we estimate that the extinction difference between the RC features corresponds to only ~5% 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. As a secondary result, we derived the extinction curve using JK_s photometry, obtaining A_J/A_Ks = 3.34 ± 0.07, consistent with previous studies of the innermost Milky Way. We find no significant variation of the extinction curve across fields or along the line of sight, within the uncertainties. The results are compatible with the secondary clump stars belonging to the spiral arm, although we cannot exclude that the population belongs to the disc.