M 82-A radio continuum and polarisation study II. Polarisation and rotation measures

Context. The composition and morphology of the interstellar medium in starburst galaxies has been well investigated, but the magnetic held properties are still uncertain. The nearby starburst galaxy M 82 provides a unique opportunity to investigate the mechanisms leading to the amplification and reduction of turbulent and regular magnetic fields.& para;& para;Aims. An investigation of the magnetic field properties in M 82 will give insight into mechanisms to generate and maintain a magnetic field as well as depolarisation mechanisms. Possible scenarios of the contribution of the magnetic field to the star-formation rate are evaluated.& para;& para;Methods. Archival data from the Very Large Array (VLA) were combined and re-reduced to cover the wavelength regime at lambda 3 cm and lambda 6 cm. Complementary data from the Westerbork Synthesis Radio Telescope (WSRT) at 418 cm and lambda 22 cm were reduced and analysed using the RM-Synthesis technique.& para;& para;Results. All observations revealed polarised emission in the inner part of the galaxy, while extended polarised emission up to a distance of 2 kpc from the disk was only detected at lambda 18 cm and lambda 22 cm. 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 M-circle dot yr(-1), which can be a significant contribution to the star-formation rate (SFR) of M 82 of similar to 13 M-circle dot yr(-1). The halo shows polarised emission, which might be the remnant of a regular disk held. Indications for a helical held in the inner part of the outflow cone are provided. The coherence length of the magnetic held in the centre could be estimated to 50 pc, which 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 held as well as a two-phase magnetic held topology were found. Electron densities in the halo (< n(e)> approximate to 0.009 cm(-3)) are similar to the ones found in the Milky Way.& para;& para;Conclusions. The magnetic held morphology is similar to the one in other nearby starburst galaxies (NGC 1569, NGC253) with possible large-scale magnetic loops in the halo and a helical magnetic held 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, but cannot be the cause for all starbursts.