2019A&A...622A...9M

Context. Low-frequency radio continuum studies of star-forming edge-on galaxies can help to further understand how cosmic-ray electrons (CRe) propagate through the interstellar medium into the halo and how this is affected by energy losses and magnetic fields.
Aims. Observations with the Very Large Array (VLA) from Continuum Haloes in Nearby Galaxies - an EVLA Survey (CHANG-ES) are combined with those with the LOw Frequency ARray (LOFAR) from the LOFAR Two-metre Sky Survey (LoTSS ) to identify the prevailing mode of cosmic-ray transport in the edge-on spiral galaxy NGC 3556.
Methods. We mapped the radio spectral index, magnetic field strength, and orientation using VLA 1.5 and 6GHz and LOFAR 144MHz data, and we fit 1D cosmic-ray propagation models to these maps using SPINNAKER (Spectral Index Numerical Analysis of K(c)osmic-ray electron radio emission) and its interactive wrapper SPINTERACTIVE.
Results. We find that the spectral index in the galactic midplane is, as expected for young CRe, α≥-0.7 and steepens towards the halo of the galaxy as a consequence of spectral ageing. The intensity scale heights are about 1.4 and 1.9kpc for the thin disc, and 3.3 and 5.9kpc for the thick disc at 1.5GHz and 144MHz, respectively. While pure diffusion cannot explain our data, advection can, particularly if we assume a linearly accelerating wind. Our best-fitting model has an initial speed of 123km/s in the galactic midplane and reaches the escape velocity at heights between 5kpc and 15kpc above the disc, depending on the assumed dark matter halo of the galaxy. This galactic wind scenario is corroborated by the existence of vertical filaments seen both in the radio continuum and in Hα in the disc-halo interface and of a large-scale reservoir of hot, X-ray emitting gas in the halo.
Conclusions. Radio haloes show the existence of galactic winds, possibly driven by cosmic rays, in typical star-forming spiral galaxies.