Doctor of Philosophy
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.
Summary for Lay Audience
Peppered throughout the interstellar medium of our Galaxy and other galaxies, young, massive stars are born from the depths of gigantic molecular clouds. These stars give off a large amount of UV light, which carves out ionized cavities around them through the irradiation of the surrounding cloud. The interface between the ionized cavity (or HII region) and the molecular cloud is referred to as a photo-dissociation region (PDR).
Within these irradiated PDRs, the physics and chemistry of its gas and dust constituents are driven by the high quantity of incident UV radiation, hence the name. The chemical species native to PDRs range from simple atomic gases to large dust grains of sub-millimeter size. Somewhere in between, we find large quantities of a peculiar group of molecules referred to as polycyclic aromatic hydrocarbons (PAHs). These PAHs are akin to such earthbound species found in soot, auto exhaust, tobacco smoke, charcoal, etc. All of which are notably highly carcinogenic.
We are able to detect emission from different gas and dust species within PDRs using astronomical observations that cover a large range in the electromagnetic spectrum. In particular, at mid-Infrared wavelengths of light, there is a prominent group of broad emission bands that are now widely accepted to result from the vibrational relaxation of PAH species that have previously absorbed a UV photon.
In this thesis, we investigate the dominant PAH emission features within four relatively nearby PDRs including: three reflection nebulae and the famous Orion Nebula. Questions we try to answer include: i) What is the effect of the UV radiation field on PAH molecular properties such as size and ionization state? ii) How do the PAH emission features vary within different types of interstellar environments? iii) Can we use these PAH emission features to quantitatively measure the physical conditions within PDRs?
Knight, Collin Matthew, "Mid-Infrared Studies of Galactic sources: Probing the Relationship Between Polycyclic Aromatic Hydrocarbons and their Physical Environment" (2020). Electronic Thesis and Dissertation Repository. 7493.