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tokens: one-to-one correspondence of the brightest H II regions and the brightest LOFAR emission neither spatially nor in flux/intensity. This may be an effect of the dust absorption, which affects the H α line flux in the disc. The number of H II regions that coincide with LOFAR emission are therefore at the front side of the galaxy. The H α bright central region is also the brightest region in radio continuum, implying efficient dust removal, probably due to a wind from this central region. There are several kiloparsec-sized radial filaments visible in the LoTSS image extending into the halo; see arrow marks in Fig. 12 . In most cases (#2, 3, 4, and 7) these filaments connect back to large H II regions in the disc of NGC 3556, or broader filaments (#1, 5, 6, and 8) connect to a large, bright region containing DIG. It is tempting to identify these filaments as magnetised chimneys from giant H II regions and more evolved, bright regions of DIG with still significant SNe activity. Despite the lower resolution and smaller signal-to-noise ratio, the overall morphology of the radial filaments extending into the halo resembles that of the starburst galaxy M82, see for example the unpublished 5 GHz map (NRAO News Release, 2014 February 3). The non-detection of a diffuse large-scale outflow in our H α data is most probably due to the relatively low sensitivity of our optical data. Still, the clear connection of non-thermal radio continuum filaments to large star-forming regions and regions of large diffuse H α emission implies an outflow/wind driven by the combined pressure from cosmic rays and hot thermal gas as result of SNe activity. Fig. 12. Sub-image measuring 9′ × 6′ of the high-resolution LoTSS tile containing NGC 3556 plotted as greyscale image; our continuum-subtracted H α image is overlaid as a pseudo-colour image (intensity scale rainbow: from low intensity = violet and blue to high intensity = red). North is up, east is left in the image. Several radio structures are marked and numbered. 4. Discussion As shown in Sect. 3 , the extent of the faint halo increases from high to low frequency. The radial extent, however, does not increase. This has been observed in many spiral edge-on galaxies (e.g. in the CHANG-ES sample) and can be interpreted as a cosmic-ray driven galactic wind ( Butsky Quinn 2018 ). Furthermore, the extent in the CHANG-ES maps is generally smaller than in the LOFAR map. This can be a result of the missing-spacing problem, where an interferometer does not “see” emission on angular scales above a certain size. For the D-array/ C -band combination, however, the largest angular scale is about 4′ in size 7 . This is comparable to the extent seen in the LOFAR map, so it is reasonable to assume that the halo would be seen in its entirety. Hence, we conclude that since the observed halo extent is smaller than in the 144 MHz map, the CRe did not propagate any further as a result of spectral ageing. As the top panel of Fig. 5 shows, the