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Probing the Ionization States of Polycyclic Aromatic Hydrocarbons via the 15-20 μm Emission Bands

We report new correlations between ratios of band intensities of the 15-20 μm 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 μm 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 μm band intensities. The 15.8 μm band correlates only with the 15-20 μm plateau and the 11.2 μm 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 μm bands; the 11.2, 15.8 μm bands and the 15-18 μm plateau; and the 11.0 and 17.4 μm bands. We also find that the spatial distribution of the 12.7, 16.4 and 17.8 μm bands can be reconstructed by averaging the spatial distributions of the cationic 11.0 μm and neutral 11.2 μm bands. We conclude that the 17.4 μm band is dominated by cations, the 15.8 μm band by neutral species, and the 12.7, 16.4 and 17.8 μm bands by a combination of the two. These results highlight the importance of PAH ionization for spatially differentiating sub-populations by their 15-20 μm emission variability.