Nearby active and starburst galaxies as seen with ISOCAM

We present a set of spectral characteristics distinguishing optically selec ted active galactic nuclei (AGNs) from starbursts in the mid-IR. In AGN spe ctra, the Unidentified Infrared Bands (UIBs) are not detected which is prob ably due to the destruction of their carriers by the UV-X radiation field. In addition, a non-negligible continuum below 10 mu m commonly associated w ith emission from very hot dust is exclusively present in AGNs. On the cont rary, starburst spectra present clearly detected UIBs with a faint underlyi ng continuum between 5-10 mu m. Based on these features, we build a mid-IR diagnostic diagram to identify emission induced by the AGN and that associa ted with the star formation activity. This diagnostic based on the mid-IR c ontinuum and the UIB intensity allows us to estimate the relative importanc e of the three components (HII regions, photo-dissociation regions (PDRs), and AGN) contained in the integrated mid-IR emission of galaxies. Our diagn ostic diagram can be applied to reveal the presence of very obscured active nuclei behind large columns of dust.