We present results of mid-infrared \lambda = 5.0 - 16.5 ~ { } \mathrm { \mu m } spectrophotometric imaging of the starburst galaxies M 82 , NGC 253 , and NGC 1808 from the ISOCAM instrument on board the Infrared Space Observatory .
The mid-infrared spectra of the three galaxies are very similar in terms of features present .
The \lambda \ga 11 ~ { } \mathrm { \mu m } continuum attributed to very small dust grains ( VSGs ) exhibits a large spread in intensity relative to the short-wavelength emission .
We find that the 15 \mu m dust continuum flux density correlates well with the fine-structure [ Ar ii ] 6.99 \mu m line flux and thus provides a good quantitative indicator of the level of star formation activity .
By contrast , the \lambda = 5 - 11 ~ { } \mathrm { \mu m } region dominated by emission from polycyclic aromatic hydrocarbons ( PAHs ) has a nearly invariant shape .
Variations in the relative intensities of the PAH features are nevertheless observed , at the 20 \% - 100 \% level .
We illustrate extinction effects on the shape of the mid-infrared spectrum of obscured starbursts , emphasizing the differences depending on the applicable extinction law and the consequences for the interpretation of PAH ratios and extinction estimates .
The relative spatial distributions of the PAH , VSG , and [ Ar ii ] 6.99 \mu m emission between the three galaxies exhibit remarkable differences .
The \la 1 ~ { } \mathrm { kpc } size of the mid-infrared source is much smaller than the optical extent of our sample galaxies and 70 \% - 100 \% of the IRAS 12 \mu m flux is recovered within the ISOCAM \leq 1.5 ~ { } \mathrm { arcmin ^ { 2 } } field of view , indicating that the nuclear starburst dominates the total mid-infrared emission while diffuse light from quiescent disk star formation contributes little .