A Principal Component Analysis of polycyclic aromatic hydrocarbon
  emission in NGC 7023
  We carried out a principal component analysis (PCA) of the fluxes of five
polycyclic aromatic hydrocarbon (PAH) bands at 6.2, 7.7, 8.6, 11.0, and 11.2
$\mu$m in the reflection nebula NGC 7023 comprising of the photodissociation
region (PDR) and a cavity. We find that only two principal components (PCs) are
required to explain the majority of the observed variance in PAH fluxes (98 %).
The first PC ($PC_{1}$), which is the primary driver of the variance,
represents the total PAH emission. The second PC ($PC_{2}$) is related to the
ionization state of PAHs across the nebula. This is consistent with the results
of a similar analysis of the PAH emission in NGC 2023. The biplots and the
correlations of PCs with the various PAH ratios show that there are two subsets
of ionic bands with the 6.2 and 7.7 $\mu$m bands forming one subset and the 8.6
and 11.0 $\mu$m bands the other. However, the distinction between these subsets
is only present in the PDR. We have also carried out a separate PCA analysis of
the PAH fluxes, this time only considering variations in the cavity. This shows
that in the cavity, $PC_{2}$ is not related to the charge state of PAHs, but
possibly to structural molecular changes.
