2007ApJ...654..844O

Mid-infrared (MIR; 12-60 µm) diffuse emission in the Galactic plane and the Carina Nebula is investigated relative to the far-infrared (FIR; ≥100 µm) emission. Observations show that the ratio of the 12 µm emission to the total FIR intensity is more or less constant in the Galactic plane but exhibits a slight decrease followed by an increase as the FIR color becomes bluer in the Carina Nebula. The constancy is compatible with predictions from models of stochastic heating of very small grains or infrared fluorescence of polycyclic aromatic hydrocarbons. The decrease can be attributed to a weakening of the unidentified infrared band emission in strong radiation fields. Contrarily, the ratio of the 25 and 60 µm emission to FIR intensity increases linearly with field strength, which is incompatible with the model predictions. The Carina Nebula data show a much bluer FIR color than the Galactic plane, whereas the ratio of MIR to FIR emission is in a range similar to that of the plane. We interpret these characteristics in terms of a superposition of emission from dust grains in various radiation field strengths. The linear increase can be accounted for by an increasing contribution from emission by dust grains in strong radiation environments, whereas the shift in FIR color between the Galactic plane and the Carina Nebula can be attributed to different contributions from grains in weak radiation fields. Other possibilities, such as the effect of multiphoton processes, variations in the incident radiation spectrum, and possible contributions from iron grains, have also been examined, but none can account for the observations consistently. Applications of the present model to external galaxies are also discussed.