The radio continuum perspective on cosmic-ray transport in external
  galaxies
  Radio continuum observations of external galaxies provide us with an
excellent outside view on the distribution of cosmic-ray electrons in the disc
and halo. In this review, we summarise the current state of what we have
learned from modelling such observations with cosmic-ray transport, paying
particular attention to the question to what extent we can exploit radio haloes
when studying galactic winds. We have developed the user-friendly framework
SPINNAKER to model radio haloes with either pure advection or diffusion,
allowing us to study both diffusion coefficients and advection speeds in nearby
galaxies. Using these models, we show that we can identify galaxies with winds
using both morphology and radio spectral indices of radio haloes. Advective
radio haloes are ubiquitous, indicating that already fairly low values of the
star formation rate (SFR) surface density ($\Sigma_{\rm SFR}$) can trigger
galactic winds. The advection speeds scale with SFR, $\Sigma_{\rm SFR}$, and
rotation speed as expected for stellar feedback-driven winds. Accelerating
winds are in agreement with our radio spectral index data, but this is
sensitive to the magnetic field parametrisation, so that constant wind speeds
cannot be ruled out either. The question to what extent cosmic rays can be a
driving force behind winds is still an open issue and we discuss only in
passing how a simple iso-thermal wind model could fit our data. Nevertheless,
the comparison with inferences from observations and theory looks promising
with radio continuum offering a complementary view on galactic winds. We finish
with a perspective on future observations and challenges lying ahead.
