The Impact of Bar-induced Non-Circular Motions on the Measurement of Galactic Rotation Curves

Abstract

We study the impact of bar-induced non-circular motions on the derivation of galactic rotation curves (RCs) using hydrodynamic simulations and observational data from the PHANGS-ALMA survey. We confirm that non-circular motions induced by a bar can significantly bias RCs derived from the conventional tilted-ring method, consistent with previous findings. The shape of the derived RC depends on the position angle difference ($Δϕ$) between the major axes of the bar and the disk in the face-on plane. For $\left|Δϕ\right|\lesssim40^\circ$, non-circular motions produce a bar-induced "dip" feature (rise-drop-rise pattern) in the derived RC, which shows higher velocities near the nuclear ring and lower velocities in the bar region compared to the true RC (${\mathrm{RC_{true}}}$). We demonstrate convincingly that such dip features are very common in the PHANGS-ALMA barred galaxies sample. Hydrodynamical simulations reveal that the "dip" feature is caused by the perpendicular orientation of the gas flows in the nuclear ring and the bar; at low $\left|Δϕ\right|$ streamlines in the nuclear ring tend to enhance $V_\mathrm{los}$, while those in the bar tend to suppress $V_\mathrm{los}$. We use a simple {\misaell} model to qualitatively explain the general trend of RCs from the tilted-ring method (${\mathrm{RC_{tilted}}}$) and the discrepancy between ${\mathrm{RC_{tilted}}}$ and ${\mathrm{RC_{true}}}$. Furthermore, we propose a straightforward method to implement a first-order correction to the RC derived from the tilted-ring method. Our study is the first to systematically discuss the bar-induced "dip" feature in the RCs of barred galaxies combining both simulations and observations.