PAHs in Galaxies: their Properties and Evolution
  I summarize the results of two recent studies, based on ISO and Spitzer mid-IR spectra of galaxies and Galactic regions, aimed at understanding the origins of the variations of the aromatic features among and inside galaxies. I show that the ratios between the most intense bands (6.2, 7.7, 8.6 and 11.3 microns) are principally sensitive to the charge of the molecules, and therefore represent a powerful diagnostic tool of the physical conditions inside the region where the emission is originating. Then, I show that the weakness of the aromatic bands, in low-metallicity environments, is a consequence of the delayed injection of their carriers, the Polycyclic Aromatic Hydrocarbons (PAHs), into the interstellar medium (ISM) of galaxies. Indeed, PAHs are believed to form in the envelopes of post-AGB stars, several hundreds of million years after the beginning of the star formation, when the system is already chemically evolved. 
F. Galliano (NASA GSFC)