M 82 – A radio continuum and polarisation study^⋆

I. Data reduction and cosmic ray propagation

¹ Astronomisches Institut der Ruhr–Universität Bochum (AIRUB), Universitätsstrasse 150, 44801 Bochum, Germany
e-mail: adebahr@astro.rub.de
² Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
³ Argelander – Institut für Astronomie (AIfA), Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 14 August 2012
Accepted: 5 March 2013

Abstract

Context. The potential role of magnetic fields and cosmic ray propagation for feedback processes in the early Universe can be probed by studies of local starburst counterparts with an equivalent star-formation rate.

Aims. In order to study the cosmic ray propagation and determine the magnetic field strength and dominant loss processes in the nearby prototypical starbursting galaxy M 82, a multi-frequency analysis at four radio wavelengths is presented.

Methods. Archival data from the Westerbork Synthesis Radio Telescope (WSRT) was reduced and a new calibration technique introduced to reach the high dynamic ranges needed for the complex source morphology. These data were combined with archival Very Large Array (VLA) data, yielding total power maps at λ3 cm, λ6 cm, λ22 cm, and λ92 cm.

Results. The data show a confinement of the emission at wavelengths of λ3/λ6 cm to the core region and a largely extended halo reaching up to 4 kpc away from the galaxy midplane at wavelengths of λ22/λ92 cm up to a sensitivity limit of 90   μJy and 1.8  mJy respectively indicating different physical processes in the core and halo regions. The results are used to calculate the magnetic field strength to 98   μG in the core region and to 24   μG in the halo regions. From the observation of ionisation losses, the filling factor of the ionised medium could be estimated to 2%. This leads to a revised view of the magnetic field distribution in the core region and the propagation processes from the core into the halo regions.

Conclusions. We find that the radio emission from the core region is dominated by very dense H ii-regions and supernova remnants, while the surrounding medium is filled with hot X-ray and neutral gas. Cosmic rays radiating at frequencies higher than 1.4 GHz suffer from high synchrotron and inverse Compton losses in the core region and are not able to reach the halo. Even the cosmic rays radiating at longer wavelengths are only able to build up the observed kpc-sized halo, when several starbursting periods are assumed where the far-infrared and radio luminosity vary by an order of magnitude. These findings, together with the strong correlation between Hα, ionised polycyclic aromatic hydrocarbons (PAH⁺), and our radio continuum data, suggest a magnetic field which is frozen into the ionised medium and driven out of the galaxy kinematically.