Published online by Cambridge University Press: 30 March 2011
A large number of galactic and extragalactic sources exhibit a mid-IR spectrum that isdominated by PAH emission. Looking at these spectra in more details reveals a strongvariability in the shape/position of the observed features, depending on the observedsource, or even the region within a source. In this article, we present the results of ananalysis that has allowed us to decompose these spectra into components having a physicalmeaning. Most, if not all PAH dominated mid-IR spectra of HII regions, PDRs,protoplanetary disks, galaxies etc. can be fitted efficiently using a combination of thesecomponents. The results of these fits provide further insight in the composition of theemitting material and the local physical conditions. In the frame of the future IR spacemissions (JWST, SPICA), this approach can be very useful to probe the physical conditionsin distant galaxies.