| Sumario: | We present a set of spectral characteristics distinguishing optically selected active galactic nuclei (AGNs) from starbursts in the mid-IR. In AGN spectra, the Unidentified Infrared Bands (UIBs) are not detected which is probably due to the destruction of their carriers by the UV-X radiation field. In addition, a non-negligible continuum below 10 μm commonly associated with emission from very hot dust is exclusively present in AGNs. On the contrary, starburst spectra present clearly detected UIBs with a faint underlying continuum between 5-10 μm. Based on these features, we build a mid-IR diagnostic diagram to identify emission induced by the AGN and that associated with the star formation activity. This diagnostic based on the mid-IR continuum and the UIB intensity allows us to estimate the relative importance of the three components (HII regions, photo-dissociation regions (PDRs), and AGN) contained in the integrated mid-IR emission of galaxies. Our diagnostic diagram can be applied to reveal the presence of very obscured active nuclei behind large columns of dust.
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