The Shape of Mid-IR PAH Bands in the Universe

A large number of galactic and extragalactic sources exhibit a mid-IR spectrum that is dominated by PAH emission. Looking at these spectra in more details reveals a strong variability in the shape/position of the observed features, depending on the observed source, or even the region within a source. In this article, we present the results of an analysis that has allowed us to decompose these spectra into components having a physical meaning. 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 these components. The results of these fits provide further insight in the composition of the emitting material and the local physical conditions. In the frame of the future IR space missions (JWST, SPICA), this approach can be very useful to probe the physical conditions in distant galaxies.