Axisymmetric Jeans modelling systematically overestimates the circular speed in the inner Milky Way

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 Vc,Jeans(R) with a reference axisymmetric circular-speed curve Vc,axi(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 ~ 1.5–2σ systematic deviation of Vc,Jeans from Vc,axi. For the 115^∘ configuration, the bias reverses sign and Vc,Jeans tends to underestimate Vc,axi. As a scaling under the usual spherical approximation, for the 25^∘ configuration a ≈ 4% bias in Vc 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.