A new mid-infrared diagnostic between AGN and starbursts

We present a new set of diagnostics which allow us to trace and classify in a statistical manner the mid-IR emission produced by active galactic nucle i (AGN) and star-forming regions. We construct a diagram based on the stren gth of the unidentified infrared band (UIB) at 6.2 mum, and the intensity o f the continuum at short (6 mum) and long wavelengths (15 mum). We interpre t the integrated mid-IR emission in late-type galaxies as resulting from th ree individual contributions coming from HII regions, diffuse/photodissocia tion regions (PDRs), and AGN. Based on this assumption, our diagnostic diag ram provides a quantitative estimate of the AGN and starburst contribution to an observed mid-IR spectrum. We show that UIB emission is very faint or absent in regions harbouring intense and hard radiation fields as in the ca se of AGN or 'pure' HII starburst regions where UIB carriers can be destroy ed by photodissociation. However, contrary to starburst spectra, typical AG N spectra present a strong hot continuum below 9 mum originating from hot d ust heated by the AGN radiation field. An extrapolation of this diagnostic towards other mid-IR observations should improve our knowledge of the AGN/s tarburst connection.