Global star formation and mid-infrared emission features in galaxies

In view of the controversy as to whether the global aromatic features in emission (AFEs) can be used as a robust tracer of the current star formation in normal galaxies, we have constructed a simple two-temperature dust emission model consisting of a cold (diffuse) component of 20K and a warm component associated with star-forming regions. Based oil a large sample of star-forming galaxies with available homogeneous IRAS infrared and SCUBA sub-mm fluxes, we show that both these dust components contribute to the global AFE emission. Only for very active star-forming galaxies (e.g., with IRAS f(v)(60 mu m)/f(v)(100 mu m) > 0.6), does the star-forming component become dominant. We show that our model predictions are consistent with the Spitzer archival data in the case of the nearby galaxy NGC 6946.