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

Abstract We study the impact of bar-induced noncircular motions on the derivation of galactic rotation curves (RCs) using hydrodynamic simulations and observational data from the PHANGS-ALMA survey. We confirm that noncircular 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 difference in position angle (Δ ϕ ) between the major axes of the bar and the disk in the face-on plane. For <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="∣" open="∣"> <mml:mrow> <mml:mo mathvariant="normal">Δ</mml:mo> <mml:mi>ϕ</mml:mi> </mml:mrow> </mml:mfenced> <mml:mo>≲</mml:mo> <mml:mn>40</mml:mn> <mml:mo>°</mml:mo> </mml:math> , noncircular 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 (RC true ). We demonstrate convincingly that such dip features are very common in the PHANGS-ALMA sample of barred galaxies. 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mfenced close="∣" open="∣"> <mml:mrow> <mml:mo mathvariant="normal">Δ</mml:mo> <mml:mi>ϕ</mml:mi> </mml:mrow> </mml:mfenced> </mml:math> streamlines in the nuclear ring tend to enhance V los , while those in the bar tend to suppress V los . We use a simple “misaligned ellipses” model to qualitatively explain the general trend of RCs from the tilted-ring method (RC tilted ) and the discrepancy between RC tilted and 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 by combining both simulations and observations.