We report new correlations between ratios of band intensities of the 15-20 \upmu m emission bands of polycyclic aromatic hydrocarbons ( PAHs ) in a sample of fifty-seven sources observed with Spitzer/IRS .
This sample includes Large Magellanic Cloud point sources from the SAGE-Spec survey , nearby galaxies from the SINGS survey , two Galactic ISM cirrus sources and the spectral maps of the Galactic reflection nebulae NGC 2023 and NGC 7023 .
We find that the 16.4 , 17.4 and 17.8 \upmu m band intensities are inter-correlated in all environments .
In NGC 2023 and NGC 7023 these bands also correlate with the 11.0 and 12.7 \upmu m band intensities .
The 15.8 \upmu m band correlates only with the 15-18 \upmu m plateau and the 11.2 \upmu m emission .
We examine the spatial morphology of these bands and introduce radial cuts .
We find that these bands can be spatially organized into three sets : the 12.7 , 16.4 and 17.8 \upmu m bands ; the 11.2 , 15.8 \upmu m bands and the 15-18 \upmu m plateau ; and the 11.0 and 17.4 \upmu m bands .
We also find that the spatial distribution of the 12.7 , 16.4 and 17.8 \upmu m bands can be reconstructed by averaging the spatial distributions of the cationic 11.0 \upmu m and neutral 11.2 \upmu m bands .
We conclude that the 17.4 \upmu m band is dominated by cations , the 15.8 \upmu m band by neutral species , and the 12.7 , 16.4 and 17.8 \upmu m bands by a combination of the two .
These results highlight the importance of PAH ionization for spatially differentiating sub-populations by their 15-20 \upmu m emission variability .