  Polycyclic Aromatic Hydrocarbons (PAHs) are carbon-based molecules resulting
from the union of aromatic rings and related species, which are likely
responsible for strong infrared emission features (3.3, 6.2, 7.7, 8.6, 11.3 and
12.7 microns). In this work, using a sample of 50 Seyfert galaxies (DL<100 Mpc)
we compare the circumnuclear (inner kpc) PAH emission of AGN to that of a
control sample of star-forming galaxies (22 luminous infrared galaxies and 30
HII galaxies), and investigate the differences between central and extended PAH
emission. Using Spitzer/InfraRed Spectrograph spectral data of Seyfert and
star-forming galaxies and newly developed PAH diagnostic model grids, derived
from theoretical spectra, we compare the predicted and observed PAH ratios. We
find that star-forming galaxies and AGN-dominated systems are located in
different regions of the PAH diagnostic diagrams. This suggests that not only
are the size and charge of the PAH molecules different, but also the nature and
hardness of the radiation field that excite them. We find tentative evidence
that PAH ratios in AGN-dominated systems are consistent with emission from
larger PAH molecules (Nc>300-400) as well as neutral species. By subtracting
the spectrum of the central source from the total, we compare the PAH emission
in the central vs extended region of a small sample of AGN. In contrast to the
findings for the central regions of AGN-dominated systems, the PAH ratios
measured in the extended regions of both type 1 and type 2 Seyfert galaxies can
be explained assuming similar PAH molecular size distribution and ionized
fractions of molecules to those seen in central regions of star-forming
galaxies (100<Nc<300).
