M 82 – A radio continuum and polarisation study

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 -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.

Magnetic field structure and halo in NGC 4631

“…Hence, this interaction may be responsible for the large spread of the values of the vertical scale heights within NGC 4631 and its larger mean values compared to those of the other six edgeon galaxies which are not strongly tidally interacting. A similar result has recently been found for M 82 (Adebahr et al 2013). -The rotation measures between λ 3.6 and 6.2 cm are characterized by a smooth large-scale distribution, indicating that NGC 4631 has a regular magnetic field.…”

“…In fact, another galaxy with strong tidal interaction is the starburst Galaxy M 82, which had already been compared with NGC 4631 several times in the literature. Recently, Adebahr et al (2013) was able to determine scale heights at four different wavelengths for this galaxy. All scale heights were smaller than the usual values and differed systematically between the northern and southern halves of M 82.…”

Modelling the cosmic ray electron propagation in M 51

“…Not only do relativistic electrons leave via diffusion but in starburst galaxies, large-scale galactic winds can also advect CREs out of the disk into the halo on a shorter timescale than the diffusion observed in M 82 (Adebahr et al 2013) and NGC 253 (Heesen et al 2009). There is no observational evidence for a global outflow or a galactic-scale wind in M 51 (Mao et al 2015).…”

Radio synchrotron spectra of star-forming galaxies

“…NGC 3034 (M82) has been the target of a large number of radio continuum studies across a large frequency range. Adebahr et al (2013) presented observations at λλ3, 6, 22, and 92 cm, using the VLA and the WSRT. Varenius et al (2015) made dedicated measurements with LOFAR to image the radio continuum of M 82 at 118 and 154 MHz.…”