Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Els Peeters


Polycyclic aromatic hydrocarbons (PAHs) are highly abundant molecules found throughout the universe. Illuminated by ultraviolet photons, they can emit up to 10% of the total power output of star-forming galaxies and are involved in many important physical and chemical processes (e.g., gas heating). They produce prominent emission bands between 3-20 μm which vary strongly in intensity and spectral profile. The origins of these variations are not fully understood, which limits the use of PAHs as (ubiquitous) diagnostic tracers of physical conditions. To characterize their variability, we studied the relative intensities and profiles of the 10-20 μm PAH bands in a varied sample of objects, including several extended Galactic sources. Our overarching conclusion is that charge is the dominant parameter in determining the strengths and/or shapes of the 10-20 μm bands. While some bands are due to a single charge state, others exhibit mixed contributions--the mixing ratios of which determine the overall band shape and thus drive the profile variations. We also investigate the nature of the surprisingly strong PAH emission observed in lines of sight toward the Galactic bulge. These sight lines are not associated with any known stellar sources. We use forbidden emission lines and spectral energy distributions to determine the physical conditions and relate these to the observed variability of the PAH emission bands, hypothesizing about possible origins of the excitation.