ISOCAM view of the starburst galaxies M 82 , NGC 253 , and NGC 1808 ⋆

We present results of mid-infrared λ = 5.0 − 16.5 µm 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 λ > ∼ 11 µm 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 µm dust continuum flux density correlates well with the fine-structure [Ar ii] 6.99 µm line flux and thus provides a good quantitative indicator of the level of star formation activity. By contrast, the λ = 5 − 11 µm 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 µm emission between the three galaxies exhibit remarkable differences. The < ∼ 1 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 µm flux is recovered within the ISOCAM ≤ 1.5 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.