The mid-infrared spectral properties of starburst galaxies and AGNs seen by ISOCAM

We present a new diagnostic tool based on mid-IR spectra (5-16 μm) to distinguish the emission triggered by an active galactic nucleus (AGN) from that associated with star formation activity. Mid-IR emission in galaxies integrated over large regions (from 100 pc to several kpc) can be understood as a mixing of three components (HII regions, diffuse/photo-dissociation regions (PDRs), and AGN). We show that AGN spectra, contrary to starburst spectra, present an important continuum below 9 μm emitted by hot dust and an absence of Unidentified Infrared Bands (UIBs) probably destroyed by photodissociation mechanism in the AGN radiation field. The diagnostic diagram derived from these features can be used to detect obscured AGNs embedded in a large concentration of dust.