CHANG-ES

Context. Cosmic rays and magnetic fields play an important role in the formation and evolution of galaxies. Radio continuum observations enable their study in the halos of edge-on galaxies. Aims. We explore the frequency dependence of the radio scale height which depends on cosmic ray transport and electron cooling. We test the influence of fundamental galaxy properties, such as the star formation rate (SFR), mass and size. Methods. We used radio continuum data of 16 edge-on galaxies from the Continuum Halos in Nearby Galaxies – an EVLA Survey (CHANG-ES). We included maps from the LOw Frequency ARray (LOFAR) at 144 MHz and from the Jansky Very Large Array (VLA) at 3 GHz with 7″ angular resolution. We extracted vertical intensity profiles within the effective radio radius and fitted beam-convolved double-exponential models to separate the thin and thick discs. For the thick radio discs, we computed their mean spectral indices and scale-height ratios between 144 MHz and 3 GHz. Results. We find a mean scale-height ratio of 1.26 ± 0.16. This is much lower than what we would expect for either cosmic ray diffusion or advection if synchrotron and inverse Compton losses dominate for the electrons. There is a moderate positive correlation between the ratio and spectral index of the thick disc: galaxies with high ratios have flat radio spectra. The ratio does not depend on any other galaxy parameter. The radio spectrum of the thick disc, as indicated by the radio spectral index, steepens with total mass (strong correlation) and flattens with SFR-to-mass surface density (moderate correlation). Conclusions. Galaxies with galactic winds have flat radio continuum spectra and large scale heights at low frequencies. This shows effective transport of cosmic rays in such systems.