  We present a focused study of radially-resolved varying PAH emission in the
low-luminosity AGN-host NGC 4138 using deep Spitzer/IRS spectral maps. Using
new model PAH spectra, we investigate whether these variations could be
associated with changes to the PAH grain size distribution due to
photodestruction by the AGN. Separately, we model the effects of the varying
radiation field within NGC 4138, and we use this model to predict the
corresponding changes in the PAH emission spectrum. We find that PAH band
ratios are strongly variable with radius in this galaxy with short-to-long
wavelength band ratios peaking in the starburst ring. The changing mix of
starlight appears to have a considerable effect on the trends in these band
ratios, and our radiation model predicts the shapes of these trends. However,
the amplitude of observed variation is ~2.5 times larger than predicted for
some ratios. A cutoff of small grains in the PAH size distribution, as has been
suggested for AGN, together with changes in PAH ionization fraction could
explain the behavior of the shorter bands, but this model fails to reproduce
longer band behaviors. Additionally, we find that short-to-long wavelength PAH
band ratios increase slightly within ~270pc of the center, suggesting that the
AGN may directly influence PAH emission there.
