of molecular gas has long been detected in the central regions of most ULIRGs [e.g. @Sanders1985; @Sanders1991] leading to high extinction of both their UV and optical radiation. As a result, since it appears that most galaxies do harbor a super-massive, though often quiescent, black hole [@Richstone1998], one would expect to find in their galactic nucleus observational evidence for a mixture of AGN [@Sanders1988] and/or strong compact starburst regions [@Condon1991] fueled by the high concentration of molecular gas [@Bryant1999]. Observations in the mid-infrared (MIR), which are less affected by absorption than shorter wavelengths [A$_{15\,\mu m}$ $\sim$A$_{V}$/70, @Mathis1990], thus provide a powerful probe of galactic central regions [@Soifer2000; @Soifer2001].

As we discussed in @Laurent2000, the integrated MIR emission in active galaxies is produced mainly by the interstellar dust which is heated directly by the ionization field from young stars or an AGN. This is in contrast to late type galaxies where the MIR (5–20$\mu$m) energy budget is dominated by the reprocessed emission of star forming regions in their disk and accounts for $\sim$15% of their luminosity [@Dale2001; @Helou2001; @Roussel2001]. However, the main difficulty in assessing the importance of the underlying physics in galactic nuclei, where the spatial resolution is typically poor, 