Constraining the Magnetic Effects on HiRotation Curves and the Need for Dark Halos

Abstract

The density profiles of dark halos are usually inferred from the rotation curves of disk galaxies based on the assumption that the gas is a good tracer of the gravitational potential of the galaxies. Some authors have suggested that magnetic pinching forces could alter significantly the rotation curves of spiral galaxies. In contrast to other studies that have concentrated on the vertical structure of the disk, here we focus on the problem of magnetic confinement in the radial direction to determine the magnetic effects on the H I rotation curves. It is shown that azimuthal magnetic fields hardly speed up H I disks of galaxies as a whole. In fact, based on virial constraints we show that the contribution of galactic magnetic fields to the rotation curves cannot be larger than ~10 km s-1 at the outermost point of H I detection, if the galaxies did not contain dark matter at all, and is up to 20 km s-1 in the conventional dark halo scenario. The procedure to estimate the maximum effect of magnetic fields is general and applicable to any particular galaxy disk. The inclusion of the surface terms, namely, the intergalactic (thermal, magnetic, or ram) pressure, does not change our conclusions. Other problems related to the magnetic alternative to dark halos are highlighted. The relevance of magnetic fields in the cuspy problem of dark halos is also discussed.