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We present the analysis of the Polycyclic Aromatic Hydrocarbon (PAH) spectraof a sample of 92 typical star forming galaxies at 0.03 < z < 0.2 observed withthe Spitzer IRS. We compare the relative strengths of PAH emission featureswith SDSS optical diagnostics to probe the relationship between PAH grainproperties and star formation and AGN activity. Short-to-long wavelength PAHratios, and in particular the 7.7-to-11.3 micron feature ratio, are stronglycorrelated with the star formation diagnostics D_n(4000) and H-alpha equivalentwidth, increasing with younger stellar populations. This ratio also shows asignificant difference between active and non-active galaxies, with the activegalaxies exhibiting weaker 7.7 micron emission. A hard radiation field asmeasured by [OIII]/H-beta and [NeIII]_15.6/[NeII]_12.8 affects PAH ratiosdifferently depending on whether this field results from starburst activity oran AGN. Our results are consistent with a picture in which larger PAH moleculesgrow more efficiently in richer media and in which smaller PAH molecules arepreferentially destroyed by AGN.

Polycyclic Aromatic Hydrocarbons in Galaxies at z~0.1: the Effect of Star Formation and AGN