Coupled Blind Signal Separation and Spectroscopic Database Fitting of
  the Mid Infrared PAH Features
  The aromatic infrared bands (AIBs) observed in the mid infrared spectrum are
attributed to Polycyclic Aromatic Hydrocarbons (PAHs). We observe the NGC
7023-North West (NW) PDR in the mid-infrared (10 - 19.5 micron) using the
Infrared Spectrometer (IRS), on board Spitzer. Clear variations are observed in
the spectra, most notably the ratio of the 11.0 to 11.2 micron bands, the peak
position of the 11.2 and 12.0 micron bands, and the degree of asymmetry of the
11.2 micron band. The observed variations appear to change as a function of
position within the PDR. We aim to explain these variations by a change in the
abundances of the emitting components of the PDR. A Blind Signal Separation
(BSS) method, i.e. a Non-Negative Matrix Factorization algorithm is applied to
separate the observed spectrum into components. Using the NASA Ames PAH IR
Spectroscopic Database, these extracted signals are fit. The observed signals
alone were also fit using the database and these components are compared to the
BSS components. Three component signals were extracted from the observation
using BSS. We attribute the three signals to ionized PAHs, neutral PAHs, and
Very Small Grains (VSGs). The fit of the BSS extracted spectra with the PAH
database further confirms the attribution to ionized and neutral PAHs and
provides confidence in both methods for producing reliable results. The 11.0
micron feature is attributed to PAH cations while the 11.2 micron band is
attributed to neutral PAHs. The VSG signal shows a characteristically
asymmetric broad feature at 11.3 micron with an extended red wing. By combining
the NASA Ames PAH IR Spectroscopic Database fit with the BSS method, the
independent results of each method can be confirmed and some limitations of
each method are overcome.
