Author ORCID Identifier

https://orcid.org/0000-0002-4727-7619

Semester

Spring

Date of Graduation

2025

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Loren Anderson

Committee Co-Chair

Duncan Lorimer

Committee Member

Duncan Lorimer

Committee Member

Larry Morgan

Committee Member

Dominic Ludovici

Abstract

The interstellar medium (ISM) plays a crucial role in the evolution of the Milky Way and other spiral galaxies, but numerous gaps remain in our collective understanding of its properties. The now-complete Green Bank Telescope (GBT) Diffuse Ionized Gas Survey (GDIGS) and the ongoing extension, the GBT Diffuse Ionized Gas Survey at Low Frequencies (GDIGS-Low), offer the opportunity to study the ionized component of the ISM at unmatched sensitivity and resolution. In this dissertation I will utilize GDIGS and GDIGS-Low data to improve our understanding of discrete regions of ionized gas. I first use GDIGS to provide improved measurements of the properties of known H II regions, including the first published radio recombination line (RRL) detections of 88 previously-unconfirmed H II regions, and discover eight new H II regions. In the same study I also identify 40 discrete clouds of ionized gas that do not appear to be H II regions, and find that 10 of these have inordinately high velocities; I label these 10 sources “Anomalous Velocity Features” (AVFs). In order to learn more about their properties and possible origin, I collect VLA C-band continuum observations of the AVFs as well as 800 MHz data from GDIGS-Low and data from the South African Radio Astronomy Observatory (SARAO) MeerKAT 1.3 GHz Galactic Plane Survey (SMGPS). I explore several possibilities for their origins, but do not have sufficient evidence for any decisive conclusions. Finally, I investigate the RRLs with full width at half maximum (FWHM) line widths less than 10 km/s detected in the GDIGS and GDIGS-Low data sets. These inordinately narrow lines indicate gas with a maximum temperature of 2200 K, which is unusually low for ionized gas. Approximately 10% of the narrow line spectra are RFI or noise and another roughly 20% are to carbon line emission from foreground clouds. Of the remaining spectra, ∼ 33% are associated with known H II re- gions. This dissertation represents an important step toward understanding of the ionized ISM.

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