The AGNS and their host galaxies as seen in the mid-infrared

We present a new diagnostic diagram based on ISOCAM spectra (5-16 mu m) to distinguish the emission induced by the active galactic nucleus (AGN) from that associated with the star formation activity. This diagnostic based on the mid-IR continuum and the Unidentified Infrared Band (UIB) intensity all ows us to estimate the relative importance of the three components (HII reg ions, diffuse/photo-dissociation regions (PDRs), and AGN) contained in gala xy mid-infrared emission. In AGN spectra, we confirm the absence of UIBs pr esumably photodissociated by the X-UV radiation field. In addition, a non-n egligible continuum below 9 mu m commonly associated with emission from hot dust is present in AGNs. A diagnostic diagram derived from these two resul ts can be used for detecting obscured AGNs embedded in a large concentratio n of dust.