Mid-Infrared Studies of Galactic sources: Probing the Relationship Between Polycyclic Aromatic Hydrocarbons and their Physical Environment
Abstract
Over the past 50 years, prominent mid-infrared (MIR) emission features from 3--20 $\mu$m have been observed ubiquitously in the interstellar medium (ISM) of Galactic and extragalactic sources. These emission features arise from the vibrational relaxation of polycyclic aromatic hydrocarbons (PAHs) after the absorption of a 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 regions (PDR) environment. We employ these variations to characterize the PAH populations in terms of properties such as degree of ionization and sizes and investigate their dependence on the physical conditions such as the FUV radiation field strength, the gas density and the gas temperature for nearby spatially resolved Galactic PDRs. We find both size and charge tend to rise with increasing radiation field strength or proximity to the illuminating source. Correlations between PAH emission features in spatially resolved sources are found to be highly dependent on the PDR morphology (i.e. edge-on versus face-on) and environmental conditions. These results are indicative of significant UV processing driving the photochemical evolution of astronomical PAH populations. We utilize observations of far-infrared (FIR) cooling lines of atoms and the FIR dust continuum emission of a nearby reflection nebula in combination with PDR models to derive maps of the physical conditions. Comparing these derived physical conditions with PAH emission characteristics at a matching spatial resolution and apertures allows us to critically test previous 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.