Dynamical Effect of Galactic Magnetic Fields on the HI Gas Rotation Curve

Magnetic fields are known to influence the thermal and structural properties of galactic gas, yet their impact on the rotational dynamics of galaxies remains largely underexplored. In this study, we examine how the magnetic arms of the spiral galaxy NGC 6946 affect its HI-traced circular gas rotation, incorporating two dark matter density models: the pseudo-isothermal (ISO) and the Navarro-FrenkWhite (NFW) profile. To assess the magnetic contribution, we employ a three-dimensional representation of the galaxy’s magnetic field and perform a χ 2 minimization to match the modeled rotation curve with observational data. Our results show that including magnetic effects yields a better fit to the HI rotation curve, particularly in the outer disk where magnetic forces become more significant. The typical amplitude of the regular magnetic field contribution to the rotation curve increases with galactocentric radius, reaching approximately 14 km s−1 in the outer gaseous disk of the galaxy NGC 6946, which corresponds to about five percent of the observed rotational velocity. These findings suggest that largescale magnetic fields play a non-negligible role in the global dynamics of spiral galaxies, especially at large radii.