Coupled blind signal separation and spectroscopic database fitting of the mid infrared PAH features

Abstract

Context. The aromatic infrared bands (AIBs) observed in the mid infrared spectrum of galactic and extragalactic sources are attributed to polycyclic aromatic hydrocarbons (PAHs). Recently, two new approaches have been developed to analyze the variations of AIBs in terms of chemical evolution of PAH species: blind signal separation (BSS) and the NASA Ames PAH IR Spectroscopic Database fitting tool. Aims. We aim to study AIBs in a photo-dissociation region (PDR) since in these regions, as the radiation environment changes, the evolution of AIBs are observed. Methods. We observe the NGC 7023-north west (NW) PDR in the mid-infrared (10-19.5 μm) using the InfraRed Spectrometer (IRS), on board Spitzer, in the high-resolution, short wavelength mode. Clear variations are observed in the spectra, most notably the ratio of the 11.0 to 11.2 μm bands, the peak position of the 11.2 and 12.0 μm bands, and the degree of asymmetry of the 11.2 μm 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. Results. 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 PAH + and PAH 0 and provides confidence in both methods for producing reliable results. Conclusions. The 11.0 μm feature is attributed to PAH + while the 11.2 μm band is attributed to PAH 0 . The VSG signal shows a characteristically asymmetric broad feature at 11.3 μm 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.

Hydrogenation and dehydrogenation of interstellar PAHs: Spectral characteristics and H2formation

“…This is apparent from the decrease with A V in the strength of the 6-9 µm stretching modes relative to the 11-15 µm CH out-of-plane bending modes (Langhoff 1996;Allamandola et al 1999;Peeters et al 2002). The signature of positively ionized species is also apparent in the broadening of the 11.25 µm band, which is in fact due to the appearance of the 11.05 µm feature related to CH out-of-plane bending modes in ionized PAHs (Rosenberg et al 2011;Boersma et al 2013). …”

“…The use of such databases has provided great insight on how different classes of PAHs can leave particular footprints in the emission at mid-IR wavelengths. In this regard, spectral variations due to changes in the ionization state of PAH molecules have been well characterized (e.g., Rosenberg et al 2011;Boersma et al 2014). Indeed, PAH band ratios such as the 6.2/11.3, 7.7/11.3 and 11.0/11.3 are now used as tracers of the ionization state of the emitting population of PAHs (Galliano et al 2008;Fleming et al 2010;Boersma et al 2014).…”

“…The combination of these studies with theoretical calculations on the IR emission of different PAH species, has allowed a characterization of the emission of PAHs as a whole (Rapacioli et al 2005;Berné et al 2007;Rosenberg et al 2011;Boersma et al 2013Boersma et al , 2014Boersma et al , 2015Andrews et al 2015). Hundreds of theoretical (and experimental) spectra of PAHs have been compiled in the NASA Ames PAH IR Spectroscopic Database (PAHdb 1 ; Bauschlicher et al 2010;Boersma et al 2014) and the Cagliari PAH Database 2 (Malloci et al 2007).…”

The Infrared Emission Bands

“…Figure from Engelbracht et al (2005). these correlations studies are now extended to included the weaker IR emission bands (e.g. Boersma et al 2010;Rosenberg et al 2011;Peeters et al 2012).…”

“…Another approach is the mathematical decomposition of PAH spectra within extended sources as a linear combination of a basis set of components which are spatially distinct (Boissel et al 2001;Rapacioli et al 2005;Berné et al 2007Berné et al , 2009Berné et al , 2011Rosenberg et al 2011). A basis set of three components is found towards RNe and the edges of clouds.…”