Transport of cosmic rays in the nearby starburst galaxy NGC 253
  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+/-30 km/s 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 a 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
Halpha- 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.
