Variations of the Mid-IR Aromatic Features inside and among Galaxies

We present the results of a systematic study of mid-IR spectra of Galactic regions, Magellanic H II regions, and galaxies of various types (dwarf, spiral, starburst), observed by the satellites ISO and Spitzer. We study the relative variations of the 6.2, 7.7, 8.6, and 11.3 μm features inside spatially resolved objects (such as M82, M51, 30 Doradus, M17, and the Orion Bar), as well as among 90 integrated spectra of 50 objects. Our main results are that the 6.2, 7.7, and 8.6 μm bands are essentially tied together, while the ratios between these bands and the 11.3 μm band vary by 1 order of magnitude. This implies that the properties of the PAHs are remarkably universal throughout our sample and that the relative variations of the band ratios are mainly controlled by the fraction of ionized PAHs. In particular, we show that we can rule out both the modification of the PAH size distribution and the mid-IR extinction as an explanation of these variations. Using a few well-studied Galactic regions (including the spectral image of the Orion Bar), we give an empirical relation between the I_6.2/I_11.3 ratio and the ionization/recombination ratio G₀/n_eT^1/2_gas, therefore providing a useful quantitative diagnostic tool of the physical conditions in the regions where the PAH emission originates. Finally, we discuss the physical interpretation of the I_6.2/I_11.3 ratio, on galactic size scales.