A new approach to the radio{far infrared correlation for non-calorimeter galaxies

New radio continuum data for a sample of 74 spiral galaxiessupportsthecalorimetermodelofV¨ olk(1989)onlyfor steep-spectrum, thin-disk, non-interacting objects which com- prise ' 30% of the sample. As the spectral indices correlate neither with far infrared surface brightness nor with average (equipartition)magneticeldstrength,thedatadisagreewiththe model for non-calorimeter galaxies of Helou & Bicay (1993). Weareabletoexplaintheradio { farinfraredcorrelationfor non-calorimetergalaxies,globallyandalsoonkiloparsecscales within galaxies, with the help of two basic relations with theav- erage volume density of the gas as the primary factor. Firstly, there is strong evidence that the strength of the equipartition magnetic eld is correlated with the volume density of the (almost) neutral gas, following a power law with an exponent ofm =0 :480:05forthegalacticaveragesofoursample.Sec- ondly, taking the thermal radio emissivity as an indicator of the average star-formation rate (SFR), we obtain a 'Schmidt law' (SFR / n ) with n = 1:4 0:3. The FIR luminosity is linearly relatedtoSFR.Finally,weassumeequipartitionbetweentheen- ergydensitiesofthemagneticeldandofthecosmicrayswhich relates the synchrotron emissivity to the eld strength. Combi- nation of these relations leads to a radio { far infrared lumi- nosity 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.