Magnetic Fields are not ignorable in the dynamics of disks
  Magnetic fields are considered to be dominant when $\epsilon_{B}\geq\epsilon_{K}$, being $\epsilon_{B}=B^{2}/8\pi$ the magnetic energy density and $\epsilon_{K}=1/2 \rho\theta^{2}$ the rotation energy density, for a conventional moderate B= 1 $\mu$G. They are considered to be negligible when $\epsilon_{B}<\epsilon_{K}$ for $B\sim 10 \mu$G. With no assumption and no theoretical calculation, we show that magnetic fields cannot be ignored in the outer parts of a galaxy like the Milky Way and in the whole disk of a dwarf galaxy. 
E. Battaner, E. Florido and A. Guijarro (Dpto. Fisica Teorica y del   Cosmos, Universidad de Granada, Spain)