Using SOFIA to Connect Astronomical Polycyclic Aromatic Hydrocarbons to their Physical Environment Conditions
Event date
Collin Knight
Event Type
Summer Series

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Over the past 50 years, dominant mid-infrared (MIR) emission features from 3-20 µm have been observed ubiquitously in the interstellar medium (ISM) of Galactic and extragalactic sources. These emission features are widely attributed to the vibrational relaxation of polycyclic aromatic hydrocarbons (PAHs) after the absorption of a single far-ultraviolet (FUV) photon. PAHs are astronomically significant in that they contain up to 15% of the cosmic carbon inventory and play an important role in the physical and chemical processes of the ISM such as, for example, the gas heating and the ionization balance. Variations in the relative strengths of the major PAH bands can be used to understand their underlying molecular properties and their interaction with the surrounding photodissociation region (PDR) environment. We investigate the dependence of PAH emission on the physical conditions such as the FUV radiation field strength, the gas density and the gas temperature for nearby spatially resolved Galactic PDRs. Correlations between PAH emission features in spatially resolved sources are found to be highly dependent on these environmental conditions. These results are indicative of significant UV processing driving the photochemical evolution of astronomical PAH populations. We utilize SOFIA FIFI-LS observations of far-infrared (FIR) cooling lines and archival Herschel PACS observations of the FIR dust continuum emission of a nearby reflection nebula, NGC 1333, in combination with PDR models to derive maps of the physical conditions. We also present preliminary results of a similar approach for the ultra-compact (UC) HII region, IRAS 12063-6259. Comparing these derived physical conditions with PAH emission characteristics at a matching spatial resolution and aperture allows us to critically test previously established relationships between PAH emission and these physical conditions. From these results, we show that these relationships also hold at a higher spatial resolution than previously obtained.

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