We present new results that have a direct bearing on the interpretation of the radio-far-infrared (FIR) relation for galaxies. By separating the thermal bremsstrahlung and synchrotron components of radio emission we have decomposed the radio-FIR relation into thermal bremsstrahlung-FIR and synchrotron-FIR relations for a sample of 31 galaxies. The new relations manifest strong correlations of their own. The thermal bremsstrahlung-FIR relation has a slope close to unity in log-log space (0.97 +/- 0.02) while the synchrotron FIR relation has a slope of 1.33 +/- 0.10, significantly steeper than unity. These results shed new light on the radio-FIR relation and explain a number of previously unresolved issues. 1. We find that both radio emission components are tightly correlated with the FIR emission for early and late-type spiral as well as irregular galaxies. It follows that any mixture of the radio components produces a tight universal radio-FIR relation. 2. At high radio frequencies, thermal bremsstrahlung can dominate the radio emission over a large range of luminosities so that the slope of the radio-FIR relation approaches that of the thermal bremsstrahlung-FIR relation and is therefore close to unity. At lower frequencies, synchrotron emission dominates and the slope approaches that of the synchrotron-FIR relation which is significantly steeper than unity. This result is consistent with the dependence of the radio-FIR slope on radio frequency found in the literature. 3. Our results are consistent with a scenario in which the FIR luminosities of galaxies scale in direct proportion to the star formation rate (SFR) as determined from the thermal bremsstrahlung luminosity. However, the synchrotron luminosities scale as ~(SFR)^1.2^, and not linearly as some previous reports have suggested.
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Price et al. (1992) studied this question.