Polycyclic Aromatic Hydrocarbon emission in Spitzer/IRS maps II: A
  direct link between band profiles and the radiation field strength
  We decompose the observed 7.7 $\mu$m polycyclic aromatic hydrocarbon (PAH)
emission complexes in a large sample of over 7000 mid-infrared spectra of the
interstellar medium (ISM) using spectral cubes observed with the Spitzer/IRS-SL
instrument. In order to fit the 7.7 $\mu$m PAH emission complex we invoke four
Gaussian components which are found to be very stable in terms of their peak
positions and widths across all of our spectra, and subsequently define a
decomposition with fixed parameters which gives an acceptable fit for all the
spectra. We see a strong environmental dependence on the inter-relationships
between our band fluxes - in the HII regions all four components are
inter-correlated, while in the reflection nebulae (RNe) the inner and outer
pairs of bands correlate in the same manner as previously seen for NGC~2023. We
show that this effect arises because the RNe maps are dominated by strongly
irradiated PDR emission, while the much larger HII region maps are dominated by
emission from regions much more distant from the exciting stars, leading to
subtly different spectral behavior. Further investigation of this dichotomy
reveals that the ratio of two of these components (centered at 7.6 and 7.8
$\mu$m) is linearly related to the UV field intensity (log G$_0$). We find that
this relationship does not hold for sources consisting of circumstellar
material, which are known to have variable 7.7 $\mu$m spectral profiles.
