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

TL;DR

This study tests whether a secondary red clump (RC2) detected in low-latitude VVV fields traces a spiral arm beyond the Galactic bar by comparing the kinematics and extinction of RC1 and RC2. Using VVV J and K_s photometry and VIRAC proper motions, the authors construct RC-specific proper-motion difference maps and extinction difference maps, finding a significant Δμ_{l*} = -0.16 ± 0.02 mas yr^{-1} and ΔA_{K_s} ≈ 0.048 mag, with RC2 generally experiencing more extinction. The extinction curve is measured as A_J/A_{K_s} = 3.34 ± 0.07 with no strong spatial variation, and N-body simulations of a Milky Way–like galaxy reproduce a similar μ_{l*} offset when two populations lie at different distances, supporting the spiral-arm interpretation. While the results favor RC2 tracing a distant structure beyond the bar, the authors cannot entirely rule out RC2 belonging to the disc. Overall, the work provides kinematic and reddening evidence for a distant spiral feature along a challenging line of sight, contributing to our understanding of the inner Milky Way's morphology.

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.

Figures (9)

• Figure 1: 2MASS $K_s$ image of the target region Skrutskie:2006te, displaying the VVV survey tiles used in this study, covering $-10^\circ < l < 10^\circ$ and $-1.5^\circ < b < 1.5^\circ$, corresponding to the area analysed by Gonzalez:2018aa. Each tile has dimensions of $1.208^\circ \times 1.487^\circ$. The scale is shown at the bottom left, and the directions for Galactic north and east are indicated at the bottom right.
• Figure 2: Upper panel: CMD of $K_s$ vs $J-K_s$, with the selection box indicated, defined through colour cuts to exclude foreground stars and ensure completeness greater than 80 %. The line within the box represents the separation derived by fitting a line to stars exhibiting 50 % membership for each of the RC features (see Sect. \ref{['sec3']}). Lower panel: CMD, with each star colour-coded according to completeness values provided in Surot:2019aa.
• Figure 3: $\mu_l*$ proper motion difference map (panel a) and associated uncertainty map (panel b), showing the difference between the mean $\mu_l*$ maps of RC2 and RC1. Colour bars indicate the proper motion values for each pixel (panel a) and their associated uncertainties (panel b). Empty pixels represent regions with an insufficient number of stars to compute a value. The position of the supermassive black hole, Sgr A$^*$, is marked by a black star in each panel, and the map sizes are indicated.
• Figure 4: $\mu_b$ proper motion difference map (panel a) and associated uncertainty map (panel b), showing the difference between the mean $\mu_b$ maps of RC2 and RC1. Colour bars indicate the proper motion values for each pixel (panel a) and their associated uncertainties (panel b). Empty pixels represent regions with an insufficient number of stars to compute a value. The position of the supermassive black hole, Sgr A$^*$, is marked by a black star in each panel, and the map sizes are indicated.
• Figure 5: Left Panel: Density distribution of stars used in the simulation analysis. The stars analogous to RC1 in red ($3 < d < 11$ kpc) and those to RC2 are shown in blue ($12 < d < 16.5$ kpc). They lie within $1\deg < |b| < 1.5\deg$. Middle Panel: $\mu_l*$ distribution for RC1 and RC2 with the median marked with solid lines. Right Panel: Stellar density distribution in $\mu_l*$ - d space. The dashed black line shows the centre of the model, while the cyan curve represents the median $\mu_l*$ in each distance bin.