ON THE USE OF FAR-INFRARED LUMINOSITY AS A STAR FORMATION INDICATOR IN GALAXIES

By comparing the H-alpha and far-infrared (FIR) luminosities for an optically selected and magnitude-limited (m(B) less-than-or-equal-to 14.5) sample of galaxies, we have found a strong nonlinearity in the log L(H-alpha)-log L(FIR) correlation, and a systematic decrease of the L(FIR)/L(H-alpha) ratio from early- to late-type spirals. We show that these trends cannot be interpreted in the frame of a single-component model for the FIR emission of galaxies, with the star formation rate as the only parameter, but are consistent with a two-component model, with the cirrus fraction as the second parameter. We found a decreasing contribution of the cirrus component to L(FIR) toward later types, going from approximately 86% for Sa galaxies to approximately 3% for Sdm galaxies. If L(FIR) is to be used as a star formation indicator in galaxies, it needs to be corrected, to first order, for the cirrus component, taking into account the morphological type of the galaxy.