Abstract We quantify systematic biases in rotation curves inferred from steady, axisymmetric Jeans modelling when the underlying stellar velocity field is non-axisymmetric. Using a high-resolution N-body/hydrodynamic simulation of an isolated Milky Way-like disk galaxy, we construct mock stellar-kinematic measurements for two observer azimuths relative to the bar. One observer is placed at a Solar-like viewing angle of 25^ from the bar major axis, and the other at 115^. For each configuration, we analyse multiple snapshots and compare the Jeans-inferred circular-speed curve V ₂, ₉₄₀₍ₒ (R) with a reference axisymmetric circular-speed curve V ₂, ₀ₗ₈ (R) defined from the azimuthally averaged (m=0) component of the gravitational field. The Jeans analysis is performed in a wedge-shaped mock observational volume that mimics limited sky coverage. For the 25^ configuration, the mean azimuthal streaming is typically higher than the azimuthally averaged expectation by 10–15~km\, s^-1, which leads to an average overestimate of the axisymmetrically defined circular speed by 4\% (corresponding to 10~km\, s^-1) in the inner disk. Across snapshots, the mean offset corresponds to a 1. 5–2 systematic deviation of V ₂, ₉₄₀₍ₒ from V ₂, ₀ₗ₈. For the 115^ configuration, the bias reverses sign and V ₂, ₉₄₀₍ₒ tends to underestimate V ₂, ₀ₗ₈. As a scaling under the usual spherical approximation, for the 25^ configuration a 4\% bias in V ₂ corresponds to an 8\% bias in the enclosed dynamical mass at fixed radius. These results imply that steady, axisymmetric Jeans modelling of Milky Way stellar kinematics can overestimate the axisymmetrically defined circular-speed curve at the percent level unless non-axisymmetric streaming is modelled explicitly or the bias is included in the error budget.
Yamaguchi et al. (Mon,) studied this question.