2006A&A...455..853M

In order to show the importance of high spatial resolution observations of extra-galactic sources when compared to observations obtained with larger apertures such as ISO, we present N-band spectra (8-13µm) of some locations in three starburst galaxies. In particular, we show the two galactic nuclei of the spiral galaxy NGC3256, the compact IR supernebula in the dwarf galaxy IIZw40 and the two brightest IR knots in the central starburst of the WR galaxy He2-10. The spectra were obtained with TIMMI2 on the ESO 3.6m telescope. An inventory of the spectra in terms of atomic fine-structure lines and molecular bands is presented. We show the value of these high spatial resolution data in constraining properties such as the extinction in the mid-IR, metallicity or stellar content (age, IMF, etc.). We have constrained the stellar content of the IR compact knot in IIZw40 by using the mid-IR fine-structure lines and setting restrictions on the nebular geometry. We have constructed a new mid-/far-IR diagnostic diagram based on the 11.2µm PAH and continuum, accessible to ground-based observations. We find that extra-galactic nuclei and star clusters observed at high spatial resolution (as is the case of the TIMMI2 observations) are closer in PAH/far-IR to compact HII regions, while galaxies observed by large apertures such as ISO are closer to exposed PDRs such as Orion. This is likely due to the aperture difference. We find a dependence between the presence of PAHs and the hardness of the radiation field as measured by the [SIV]/[NeII] ratio that may be explained by the PAH-dust competition for FUV photons or the relative contribution of the different phases of the interstellar medium.