Mid-infrared diagnostics to distinguish AGNs from starbursts

We present new mid-infrared (MIR) diagnostics to distinguish emission of ac tive galactic nuclei (AGN) from that originating in starburst regions. Our method uses empirical spectroscopic criteria based on the fact that MIR emi ssion from star forming or active galaxies arises mostly from HII regions, photo-dissociation regions (PDRs) and AGNs. The analysis of the strength of the 6.2 mu m Unidentified Infrared Band (UIB) and the MIR continuum shows that UIBs are very faint or absent in regions harboring the intense and har d radiation fields of AGNs and pure HII regions, where the UIB carriers cou ld be destroyed. The MIR signature of AGNs is the presence of an important continuum in the 3-10 mu m band which originates from very hot dust heated by the intense AGN radiation field. Using these two distinct spectral prope rties found in our MIR templates, we build diagnostic diagrams which provid e quantitative estimates of the AGN, PDR and HII region contribution in a g iven MIR spectrum. This new MIR classification can be used to reveal the pr esence of AGNs highly obscured by large columns of dust.