  In order to analyze the importance of the star formation rate in generating
and amplifying magnetic fields in the interstellar medium we obtained deep
total power and polarization maps at 4.85 GHz of three late type spiral
galaxies NGC4236, NGC4656 and IC2574 using the 100-m Effelsberg radio
telescope. This was accompanied by imaging in the Halpha line. We also observed
these objects at 1.4 GHz to obtain their integrated fluxes at this frequency
and to determine their radio spectra. All galaxies were found to possess weak
but detectable total power emission at 4.85 GHz, coincident with regions of
recent star formation as traced by bright Halpha regions. The surface
brightness of the radio-strongest object of our sample (NGC4656) is comparable
to the radio-weakest objects in a sample of more than 50 normally star-forming
spiral galaxies for which measurements at 4.8 GHz with the Effelsberg radio
telescope are available. The fractional polarization of the 2 galaxies of our
sample is less than 2%, significantly lower than for spiral galaxies of
intermediate types, suggesting that the magnetic fields are not only weaker,
but also less ordered than in spiral galaxies. The radio spectra of galaxies in
our small sample are indicative of a substantial fraction of thermal emission,
with a higher thermal fraction than in spirals with high star formation rates
(SFR), while the nonthermal emission in our sample is relatively weak compared
to spiral galaxies. We propose an equipartition model where the nonthermal
emission increases ~SFR^(~1.4) and the ratio of nonthermal to thermal emission
increases ~SFR^(~0.4). The objects of our sample still follow the radio-FIR
correlation of surface brightness of the total emission, extending it towards
the lowest values measured so far.
