The heating of dust in starburst galaxies: The contribution of the nonionizing radiation

The IUE UV and optical spectra and the far-infrared (FIR) IRAS flux densities of a sample of starburst and blue compact galaxies are used to investigate the relationship between dust obscuration and dust emission. The amount of dust obscuration at UV wavelengths correlates with the FIR-to-blue ratio; and an analysis of the correlation indicates that not only the ionizing but also the nonionizing radiation contribute to the FIR emission. The amount of UV and optical energy lost to dust obscuration accounts for most of the cool dust FIUR emission and for about 70% of the warm dust FIR emission. The remaining 30% of the warm dust FIR flux is probably due to dust emission from regions of star formation which are embedded in opaque giant molecular clouds and do not contribute to the integrated UV and optical spectrum. The use of the FIR emission as an indicator of high-mass star formation rate in star-forming galaxies can be problematic, since the contribution to the FIR flux from cool dust emission heated by relatively old stars is nonnegligible.