Using the Cyanide Radical to Trace Dense Molecular Gas in Nearby Star-Forming Galaxies

Abstract

Dense molecular gas is the fuel for star formation in galaxies. Empirical scaling relations show that dense gas mass correlates nearly linearly with star formation rate in normal and star-forming galaxies. To understand the connection between dense gas and star formation, astronomers have previously relied on observations of molecules like hydrogen cyanide (HCN). Recently, it has been shown that the cyanide radical (CN) can be used to study dense gas in a similar way to HCN. In this thesis, I investigate the properties of dense molecular gas using new and archival observations of CN and HCN from the Atacama Large Millimeter/Submillimeter Array (ALMA) for Ultraluminous and Luminous Infrared Galaxies (U/LIRGs). I begin with a multi-transition line study of CN and HCN to compare the excitation conditions of these two molecules in a sample of three galaxies: IRAS 13120-5453, NGC 3256, and NGC 7469. I find variations between individual lines of each molecule which are connected with regions of enhanced star formation or the presence of an active galactic nuclei. I then focus on using CN as a tracer of dense gas and dense gas fraction when compared with carbon monoxide (CO). I measure the CN/CO intensity ratio in a sample of 16 galaxies and find that CN/CO is higher, on average, in ULIRGs compared to LIRGs. LIRGs have a larger spread in CN/CO ratios compared to ULIRGs, which I attribute to their variation in star formation, AGN, and morphological properties. Finally, I use the CN/CO ratio to estimate the dense gas fraction and find that it correlates with star formation rate and hard X-ray luminosity at the location of peak X-ray emission. The results of this thesis imply that CN can be used as a tool to study the physical and chemical properties of dense gas in extreme star-forming galaxies.