STAR-FORMATION HISTORIES AND THE MASS-NORMALIZED FIR RADIO CORRELATION IN LATE-TYPE GALAXIES/

We study the correlation between the mass-normalized FIR (40-120 mu m) and radio continuum (at 1.49GHz) luminosities for a sample of 114 nor mal nearby late-type galaxies. The correlation, which spans 2-3 orders of magnitude, is found to be quite strong with a correlation coeffici ent of 0.87, demonstrating that the well-known correlation between the FIR and radio emissions of galaxies is not merely a mass-scaling ('ri chness') effect. Adopting the hypothesis that the basic reason of the FIR/radio correlation is the star-formation activity which ubiquitousl y exists in late-type galaxies, the disturbance (i.e. the nonlinearity and the scatter) of the correlation due to variations of star-formati on history is estimated using a simple theoretical model, which shows that the nonlinearity and a large part of the scatter of the correlati on can be explained by this effect. Applying this model to the median values of the mass-normalized FIR luminosities, we suggest that galaxi es of different morphological type have systematically different star- formation histories on time scales of 10(10) years, consistent with pr evious optical studies.