Kinematic diagnostics for non-axisymmetry in the Milky Way’s nuclear stellar disc

¹ Como Lake centre for AstroPhysics (CLAP), DiSAT, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
² Institute of Space Sciences & Astronomy, University of Malta, Msida MSD 2080, Malta
³ Jeremiah Horrocks Institute, University of Lancashire, Preston PR1 2HE, UK
⁴ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
⁶ Department of Physics, Engineering Physics, and Astronomy, Queen’s University, Kingston, ON K7L 3N6, Canada
⁷ Université Côte d’Azur, Observatoire de la Côte d’Azur, Laboratoire Lagrange, CNRS, Blvd de l’Observatoire, 06304 Nice, France

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

Received: 22 December 2025
Accepted: 19 March 2026

Abstract

There is now strong evidence that the Milky Way (MW) hosts a nuclear stellar disc (NSD). However, whether the NSD is purely axisymmetric or contains a nuclear bar remains unresolved. Since approximately 50% of barred galaxies with MW-like mass in the local Universe host a nuclear bar, investigating whether the MW hosts one is of interest. We conducted a systematic analysis to identify robust kinematic diagnostics capable of determining whether the MW hosts a nuclear bar. Using N-body simulations, we explored the kinematic signatures indicative of a nuclear bar. Using the phase-space coordinates longitude (ℓ) and latitude (b), proper motions (μ_ℓ and μ_b), and line-of-sight velocity (v_los), we tested various diagnostics assuming different nuclear bar orientations. We also evaluated how sample size, dust extinction, and bar amplitude influence the efficacy of the diagnostics. We identify two independent kinematic diagnostics capable of revealing a nuclear bar in the MW: (1) the vertex deviation, l_v, of the (v_ℓ − v_los) velocity ellipse and (2) the asymmetry in the μ_ℓ versus ℓ distribution. While both are impacted by the sample size and extinction, the vertex deviation proves more robust, especially when combining stars from multiple observational fields. We also assessed the correlation between the line-of-sight velocity and the h₃ Gauss-Hermite moment (‘skewness’) of the line-of-sight velocity but find no clear distinction between an NSD and a nuclear bar based on this metric. Our results suggest that data from the current KMOS survey may allow a marginal detection of a nuclear bar using the vertex deviation method. A companion paper provides further validation and detailed analysis of this approach. Nonetheless, future surveys will provide the high-quality data necessary to fully exploit the diagnostics outlined in this study.