Abstract
Star-forming regions are complex systems that impact and are impacted by the surrounding environment. These regions host intricate physical and chemical processes which have marked effects on the molecular gas that serves as a reservoir for star formation. From before their births to after their deaths, stars inject energy into the surrounding interstellar medium (ISM) via feedback processes, impacting the gas and dust leftover from their formation. While this material may return to the ISM and fuel future generations of star formation, the influence of these feedback mechanisms may have long-lasting effects on the evolution of baryon matter in galaxies. In this dissertation, I will utilize molecular gas measurements toward star-forming regions to better understand the evolution of star formation in galaxies. I will investigate the processes that contribute to the kinematic structure of star-forming regions in our own galaxy, as well as examine the impact of star formation feedback on molecular gas in the extreme environments found in the centers of nearby galaxies. By examining star formation across a range of scales and conditions, I will quantify the ability of molecular gas measurements to characterize the nature and evolution of star formation in galaxies.
In Chapter 2, I investigate two star-forming cores, one an isolated system and one containing multiple protostars, in the nearby Galactic Perseus molecular cloud. Using N2H+ to trace the cold, dense gas leftover from the formation of these cores, I analyze this molecule's hyperfine structure to constrain the kinematics and angular momentum in these protostellar cores. While previous studies suggested the kinematic structure in cores was inherited from the rotation of the cores' parent clouds, my investigation reveals that the velocity structure of protostellar cores is instead a function of the cores' environments. On scales greater than ~1000 AU, the density distribution of a molecular cloud is likely the primary contributor to a core's kinematic structure.
Chapters 3, 4, and 5 focus on the center of the nearby nuclear starburst galaxy NGC 253. Starburst systems feature intense feedback mechanisms, making them excellent laboratories to study the impact of stellar feedback on molecular gas. In Chapter 3, I introduce a gas parameter inference algorithm that I use to characterize the impact of these feedback mechanisms on the dense molecular gas that will form stars. I use multi-transition measurements of HCN and HNC to infer the gas conditions in the NGC 253 Central Molecular Zone (CMZ), pioneering a new method to constrain the cosmic-ray ionization rate, an important measure of stellar feedback, at high resolution in other galaxies.
Chapter 4 builds on my work from Chapter 3 by expanding this analysis within the NGC 253 CMZ. I introduce machine learning techniques that increase the efficiency of the parameter inference algorithm, allowing me to fully map the gas conditions in NGC 253 for the first time. This study reveals the complexities of interpreting molecular emission in extreme environments and provides a template for disentangling the impacts of various feedback mechanisms on HCN and HNC emission in star-forming regions. Additionally, I discuss the impacts of such intense feedback on the flow of baryonic matter and the evolution of galaxies. This work motivates a need to further quantify both the utility and limitations of molecular emission as tracers of gas conditions in high-resolution star-forming environments.
To address this need, in Chapter 5 I test how the strength and number of HCN and HNC transitions impact the gas parameter inference process. While single transitions have been used to infer the characteristics of star-forming gas in the past, I show that at high resolution, many physical processes can affect molecular emission. Multiple transitions that span a range of upper-state energies are required to disentangle the competing and complementary effects of star formation on molecular gas emission. I provide benchmarks and recommendations for observers to consider when examining molecular gas in star-forming regions.
