by Volker Heesen (University of Southampton, UK)

Radio haloes in nearby edge-on galaxies show the presence of cosmic-ray electrons and magnetic fields in them. As an example for this, today’s image shows the radio emission and magnetic field orientation in two nearby edge-on galaxies, a result of reducing ~500 hours of archival and un-published ATCA data. Observations were carried out at 22 and 6 cm and the radio spectral indices were analysed with cosmic-ray transport models. The study shows that NGC 7090 is an advection dominated galaxy with exponential intensity and linear spectral index profiles and NGC 7462 is a diffusion dominated galaxy with Gaussian intensity and parabolic spectral index profiles. This means that NGC 7090 harbours a galactic outflow or wind, where cosmic rays can partially escape which is known as a non-calorimetric halo. On the other hand, it is debatable whether NGC 7462 possesses a `bona fide’ radio halo at all. Details of this study can be found in Heesen et al. (2016).

Figure caption. — Left panels: 20 cm radio continuum emission (exponential contours, starting at 3 sigma) overlaid on Balmer H-alpha emission. Blue ellipses show the optical extent of the disc. — Right panels: 6 cm polarized intensity (2,4 and 6 sigma) overlaid on the rotation measure between 20 and 6 cm. Vectors show the magnetic field orientation at 6 cm, not corrected for Faraday rotation.