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 6 GHz and LOFAR 144 MHz 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 , \alpha \approx - 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 1.9 kpc for the thin disc , and 3.3 and 5.9 kpc for the thick disc at 1.5 GHz and 144 MHz , respectively .
While pure diffusion can not explain our data , advection can , particularly if we assume a linearly accelerating wind .
Our best-fitting model has an initial speed of 123 km s ^ { -1 } in the galactic midplane and reaches the escape velocity at heights between 5 kpc and 15 kpc 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 H \alpha 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 .