Constraining the Magnetic Effects on HI Rotation 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 which have concentrated in the vertical structure of the disk, here we focus on the problem of magnetic confinement in the radial direction to bound the magnetic effects on the HI rotation curves. It is shown that azimuthal magnetic fields can hardly speed up the HI disk 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 at the outermost point of HI detection, if the galaxies did not contain dark matter at all, and up to 20 km/s 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 with the magnetic alternative to dark halos are highlighted. The relevance of magnetic fields in the cuspy problem of dark halos is also discussed.

arXiv comment12 pages, 1 figure, accepted for publication in The Astrophysical Journal

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• Are rotation curves in NGC 6946 and the Milky Way magnetically supported?
• Global disk model for galaxies NGC 1365, NGC 6946, NGC 7793, UGC 6446
• The Dark Halo of NGC 5963 as a Constraint on Dark Matter Self-Interaction at the Low Velocity Regime
• Magnetic fields: Impact on the rotation curve of the Galaxy
• Galaxy rotation curves: the effect of j x B force
• Influence of Magnetic Fields on the Gas Rotation in the Galaxy $NGC6946$
• The effect of magnetic field on the inner Galactic rotation curve