CHANG-ES XII: A LOFAR and VLA view of the edge-on star-forming galaxy
  NGC 3556
  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. 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. 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. We find that the spectral index in the
galactic midplane is, as expected for young CRe, alpha ~ -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 cannot 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. Radio haloes show the existence of galactic winds,
possibly driven by cosmic rays, in typical star-forming spiral galaxies.
