1997A&A...320...54N

New radio continuum data for a sample of 74 spiral galaxies supports the calorimeter model of Voelk (1989A&A...218...67V) only for steep-spectrum, thin-disk, non-interacting objects which comprise ≃30% of the sample. As the spectral indices correlate neither with far infrared surface brightness nor with average (equipartition) magnetic field strength, the data disagree with the model for non-calorimeter galaxies of Helou & Bicay (1993ApJ...415...93H). We are able to explain the radio - far infrared correlation for non-calorimeter galaxies, globally and also on kiloparsec scales within galaxies, with the help of two basic relations with the average volume density of the gas as the primary factor. Firstly, there is strong evidence that the strength of the equipartition magnetic field is correlated with the volume density ρ of the (almost) neutral gas, following a power law with an exponent of m=0.48±0.05 for the galactic averages of our sample. Secondly, taking the thermal radio emissivity as an indicator of the average star-formation rate (SFR), we obtain a `Schmidt law' (SFR∝ρⁿ) with n=1.4±0.3. The FIR luminosity is linearly related to SFR. Finally, we assume equipartition between the energy densities of the magnetic field and of the cosmic rays which relates the synchrotron emissivity to the field strength. Combination of these relations leads to a radio - far infrared luminosity correlation with a power-law exponent of x=1.3±0.3, very close to what is observed (x=1.25±0.08). Forthcoming ISO satellite data can be used as a test of our approach.