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

TL;DR

This study shows that steady, axisymmetric Jeans modelling can systematically overestimate the Milky Way's inner circular speed due to non-axisymmetric stellar streaming, with biases up to 4%.

Contribution

It quantifies the bias in Jeans modelling caused by non-axisymmetric stellar motions using high-resolution galaxy simulations.

Findings

Jeans analysis overestimates circular speed by about 4% at 25° observer angle.

Bias reverses sign at 115° observer angle, underestimating the circular speed.

A 4% bias in circular speed leads to an 8% bias in enclosed mass estimates.

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 $25^\circ$ from the bar major axis, and the other at $115^\circ$. For each configuration, we analyse multiple snapshots and compare the Jeans-inferred circular-speed curve, $V_{\rm c,Jeans}(R)$, with a reference axisymmetric circular-speed curve, $V_{\rm c,axi}(R)$, 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 $25^\circ$ configuration, the mean azimuthal streaming is typically higher than the azimuthally averaged expectation by $\approx 10$--$15~\mathrm{km\,s^{-1}}$, which leads to an average overestimate of the axisymmetrically defined circular speed by $\approx 4\%$ ($\approx 10~\mathrm{km\,s^{-1}}$) in the inner disk. Across snapshots, the mean offset corresponds to a $\sim 1.5$--$2\sigma$ systematic deviation of $V_{\rm c,Jeans}$ from $V_{\rm c,axi}$. For the $115^\circ$ configuration, the bias reverses sign and $V_{\rm c,Jeans}$ tends to underestimate $V_{\rm c,axi}$. Under the usual spherical approximation, a $\approx 4\%$ bias in $V_{\rm c}$ corresponds to an $\approx 8\%$ 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 included in the error budget.