Date of Graduation


Document Type


Degree Type



Eberly College of Arts and Sciences


Physics and Astronomy

Committee Chair

Loren Anderson

Committee Co-Chair

Dana Balser

Committee Member

Dana Balser

Committee Member

Paul Cassak

Committee Member

Duncan Lorimer

Committee Member

D.J. Pisano


The interstellar medium (ISM) of our Galaxy contains low-density diffuse ionized gas known as the warm ionized medium (WIM). O- and B-type stars emit large amounts of ionizing radiation and it is believed that a fraction of this radiation escapes from their fully ionized HII regions and into the ISM where it is responsible for maintaining the ionization of the WIM. Here we aim to better understand how the radiation produced by OB stars is able to leak from the HII regions, how the radiation field changes during this process, and how the radiation affects the ambient ISM. Using Green Bank Telescope radio recombination line data of a subset of Galactic HII regions, we show that the morphology of the photodissociation region surrounding an HII region strongly affects the amount of leaking radiation. We compute a leaking photon fraction of 15±5% for the compact HII region NGC7538 and argue that more luminous HII regions likely have a greater effect on maintaining the ionization of the WIM. The N(4He+)/N(H+) abundance ratio decreases with distance from most observed regions, indicating that helium-ionizing photons are being absorbed within the HII region ionization fronts. We find that WIM emission toward the first Galactic quadrant is correlated both with the locations of luminous HII region complexes and with the intensity of diffuse 8μm emission caused by excitation of polycyclic aromatic hydrocarbons (PAHs). This suggests that the soft-ultraviolet photons required to excite PAHs have the same origin as the more energetic radiation maintaining the WIM.