  The relation between the resolved star formation rate per unit area and the
non-thermal radio continuum emission is studied in 21 Virgo cluster galaxies
and the two nearby spiral galaxies, NGC6946 and M51. For the interpretation and
understanding of our results we used a 3D model where star formation, 2D cosmic
ray (CR) propagation, and the physics of synchrotron emission are included.
Based on the linear correlation between the star formation rate per unit area
and the synchrotron emission and its scatter radio-bright and radio-dim regions
can be robustly defined for our sample of spiral galaxies. We identified CR
diffusion or streaming as the physical causes of radio-bright regions of
unperturbed symmetric spiral galaxies as NGC6946. We identified the probable
causes of radio-bright regions in several galaxies as CR transport, via either
gravitational tides (M51) or galactic winds (NGC4532) or ram pressure stripping
(NGC4330 and NGC4522). Three galaxies are overall radio-dim: NGC4298, NGC4535,
and NGC4567. Based on our model of synchrotron-emitting disks we suggest that
the overall radio-dim galaxies have a significantly lower magnetic field than
expected by equipartition between the magnetic and turbulent energy densities.
Radio-bright regions frequently coincide with asymmetric ridges of polarized
radio continuum emission, and we found a clear albeit moderate correlation
between the polarized radio continuum emission and the radio/SFR ratio. When
compression or shear motions of the interstellar medium (ISM) are present in
the galactic disk, the radio-bright regions are linked to the commonly observed
asymmetric ridges of polarized radio continuum emission and represent a useful
tool for the interaction diagnostics. Based on our results, we propose a
scenario for the interplay between star formation, CR electrons, and magnetic
fields in spiral galaxies.
