  This paper investigates the physical reasons for the existence, the tightness
and the near universality of the FIR/radio correlation for late-type field
galaxies. We develop theoretical models for the radio and far-infrared (FIR)
emission of normal galaxies and study the influence of their main parameters on
the ratio of the two emissions. In addition, data are used from a sample of 114
late-type galaxies which allow an estimate of the mean energy densities of the
radiation field and the magnetic field, and of the dust opacity. These data
reveal, for the first time, a reasonably good, linear correlation between the
energy density of the radiation field and the energy density of the magnetic
field. Furthermore we find that on average the galaxies are marginally
optically thick for the non-ionizing UV light. Including their extended
magnetic halos, galaxies are also found to be on average marginally optically
thick regarding the radiative energy losses of the Cosmic Ray electrons.
Combining these semi-empirical results with the theoretical emission models we
show that in the optically thick case the linear correlation between the energy
densities of the radiation field and the magnetic field is a necessary
condition for the existence of the observed FIR/radio correlation. Furthermore
we show that the individual dispersion of the FIR to radio continuum ratio
caused by uncertainties of any single one of the parameters considered is
significantly less than the empirical dispersion of the FIR/radio correlation.
The radio and the FIR emission are mainly determined by their sources, i.e.
massive supernova precursor stars, because galaxies act in a reasonable
approximation as calorimeters for the stellar UV radiation and for the energy
flux of the Cosmic Ray electrons.
