M82 - A radio continuum and polarisation study II. Polarisation and
  rotation measures
  The composition and morphology of the interstellar medium in starburst
galaxies has been well investigated, but the magnetic field properties are
still uncertain. The nearby starburst galaxy M82 provides a unique opportunity
to investigate the mechanisms leading to the amplification and reduction of
turbulent and regular magnetic fields. Possible scenarios of the contribution
of the magnetic field to the star-formation rate are evaluated. Archival data
from the VLA and WSRT were combined and re-reduced to cover the wavelength
regime between 3cm and 22cm. All observations revealed polarised emission in
the inner part of the galaxy, while extended polarised emission up to a
distance of 2kpc from the disk was only detected at 18cm and 22cm. The
observations hint at a magnetised bar in the inner part of the galaxy. We
calculate the mass inflow rate due to magnetic stress of the bar to 7.1 solar
masses per year, which can be a significant contribution to the star-formation
rate of M82 of approximately 13 solar masses per year. The halo shows polarised
emission, which might be the remnant of a regular disk field. Indications for a
helical field in the inner part of the outflow cone are provided. The coherence
length of the magnetic field in the centre is similar to the size of giant
molecular clouds. Using polarisation spectra more evidence for a close coupling
of the ionised gas and the magnetic field as well as a two-phase magnetic field
topology were found. Electron densities in the halo are similar to the ones
found in the Milky Way. The magnetic field morphology is similar to the one in
other nearby starburst galaxies with possible large-scale magnetic loops in the
halo and a helical magnetic field inside the outflow cones. The special
combination of a magnetic bar and a circumnuclear ring are able to
significantly raise the star-formation rate in this galaxy by magnetic braking.
