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Based on theoretical spectra computed using Density Functional Theory westudy the properties of Polycyclic Aromatic Hydrocarbons (PAH). In particularusing bin-average spectra of PAH molecules with varying number of carbons weinvestigate how the intensity of the mid-infrared emission bands, 3.3, 6.2, 7.7and 11.3 microns, respond to changes in the number of carbons, charge of themolecule, and the hardness of the radiation field that impinges the molecule.We confirm that the 6.2/7.7 band ratio is a good predictor for the size of thePAH molecule (based on the number of carbons present). We also investigate theefficacy of the 11.3/3.3 ratio to trace the size of PAH molecules and note thedependence of this ratio on the hardness of the radiation field. While theratio can potentially also be used to trace PAH molecular size, a betterunderstanding of the impact of the underlying radiation field on the 3.3microns feature and the effect of the extinction on the ratio should beevaluated. The newly developed diagnostics are compared to band ratios measuredin a variety of galaxies observed with the Infrared Spectrograph on board theSpitzer Space Telescope. We demonstrate that the band ratios can be used toprobe the conditions of the interstellar medium in galaxies and differentiatebetween environments encountered in normal star forming galaxies and ActiveGalactic Nuclei. Our work highlights the immense potential that PAHobservations with the James Webb Space Telescope will have on our understandingof the PAH emission itself and of the physical conditions in galaxies near andfar.

The properties of Polycyclic Aromatic Hydrocarbons in galaxies: constraints on PAH sizes, charge and radiation fields