Radio halos require the coexistence of extra-planar cosmic rays and magnetic fields .
Because cosmic rays are injected and accelerated by processes related to star formation in the disk , they have to be transported from the disk into the halo .
A vertical large-scale magnetic field can significantly enhance this transport . We observed NGC 253 using radio continuum polarimetry with the Effelsberg and VLA telescopes . The radio halo of NGC 253 has a dumbbell shape with the smallest vertical extension near the center .
With an estimate for the electron lifetime , we measured the cosmic-ray bulk speed as 300 \pm 30 km s ^ { -1 } which is constant over the extent of the disk .
This shows the presence of a “ disk wind ” in NGC 253 .
We propose that the large-scale magnetic field is the superposition of a disk ( r, \phi ) and halo ( r,z ) component .
The disk field is an inward-pointing spiral with even parity .
The conical ( even ) halo field appears in projection as an X-shaped structure , as observed in other edge-on galaxies . Interaction by compression in the walls of the superbubbles may explain the observed alignment between the halo field and the lobes of hot H \alpha - and soft X-ray emitting gas .
The disk wind is a good candidate for the transport of small-scale helical fields , required for efficient dynamo action , and as a source for the neutral hydrogen observed in the halo .