Abstract

The Milky Way hosts a flattened nuclear stellar disc (NSD) that dominates the gravitational potential in the inner few hundred parsecs, but whether the NSD is purely axisymmetric or contains a nuclear bar remains an open question. We tested for the presence of a nuclear bar with kinematic diagnostics, combining positions and line-of-sight velocities from the KMOS NSD survey with proper motions from VIRAC2 to construct the ( v ℓ , v los ) velocity ellipse. We measured the vertex deviation ( ℓ v ) and the anisotropy (β) of the velocity ellipse for a number of sub-samples after applying quality cuts designed to minimise contamination from large-scale bar stars. For our primary sample, comprising stars within | l | < 0.9°, −0.4° < b < 0.25°, and [Fe/H] > −0.3, we measured a negative vertex deviation, l v = −54.8 −14.8 +13.1° , with moderate anisotropy, β = 0.16 −0.05 +0.08 . A subsample restricted to the innermost four fields yields l v = −64.3 −12.2 +12.1° with higher anisotropy, β = 0.38 −0.07 +0.12 . The direction of maximum velocity dispersion is oriented along Galactic longitude, contrary to what is observed in samples dominated by large-scale bar stars, where line-of-sight dispersion dominates. We verified the robustness of these signatures against extinction-driven photometric incompleteness, primary-bar contamination, and the choice of metallicity threshold used to suppress bar interlopers; the kinematic measurements remain stable in all cases. The signatures are inconsistent with an NSD that is axisymmetric or oriented orthogonally to the primary bar but match expectations for a nuclear bar oriented at α ≈ 60°-75° to the Sun-Galactic centre line, with its near side pointing towards positive Galactic longitude. While definitive confirmation awaits the larger and more precise samples expected from upcoming surveys, our results provide the first kinematic indication of a possible nuclear bar in the Milky Way.