M 82 – A radio continuum and polarisation study
II. Polarisation and rotation measures
1 Astronomisches Institut der Ruhr-Universität Bochum (AIRUB), Universitätsstrasse 150, 44801 Bochum, Germany
2 Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
3 ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands
4 Argelander-Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
5 CSIRO Astronomy and Space Science, 26 Dick Perry Avenue, Kensington WA 6151, Australia
6 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
Received: 30 August 2016
Accepted: 18 September 2017
Context. 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 M 82 provides a unique opportunity to investigate the mechanisms leading to the amplification and reduction of turbulent and regular magnetic fields.
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.
Methods. Archival data from the Very Large Array (VLA) were combined and re-reduced to cover the wavelength regime at λ3 cm and λ6 cm. Complementary data from the Westerbork Synthesis Radio Telescope (WSRT) at λ18 cm and λ22 cm were reduced and analysed using the RM-Synthesis technique.
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 λ18 cm and λ22 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⊙ yr-1, which can be a significant contribution to the star-formation rate (SFR) of M 82 of ~ 13 M⊙ yr-1. 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 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 field as well as a two-phase magnetic field topology were found. Electron densities in the halo (⟨ ne ⟩ ≈ 0.009 cm-3) are similar to the ones found in the Milky Way.
Conclusions. The magnetic field morphology is similar to the one in other nearby starburst galaxies (NGC 1569, NGC 253) 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, but cannot be the cause for all starbursts.
Key words: galaxies: individual: M 82 / galaxies: starburst / galaxies: magnetic fields / galaxies: halos / techniques: polarimetric
© ESO, 2017