Spatially resolved study of the molecular isotopologue 13CO in nearby galaxies

Observations of 13CO are indispensable for probing physical conditions in the molecular interstellar medium (ISM) where the most commonly used 12CO emission often saturates, at the cost of weaker emission that is difficult to observe. The line intensity ratio 12CO(1-0)/13CO(1-0) (hereafter R) can be used to trace variations in optical depths determined by the physical conditions, although it can also be influenced by chemical processes. In this thesis, I investigate the 13CO emission and the line ratio R on kiloparsec (kpc) scales for a wide variety of galaxies: 11 galaxies from the CARMA STING survey and 42 galaxies from the CARMA EDGE survey. Using the combination of millimeter-wave interferometry and rich existing auxiliary data for these surveys, I study the molecular gas properties in relation to the stellar population and other ISM components within galaxies and to the global parameters of the galaxies. Among our sample, we find that the R values are higher in the interacting galaxies in the isolated galaxies. However, we do not find a strong dependence of R on global properties such as galaxy type, stellar mass, inclination, or size of the host galaxy. On kilosparsec-scales, lower R values are usually found in regions with weaker 12CO. We attribute this apparent trend to a bias against measuring large values when 12CO is weak. Limiting our analysis to the 12CO-bright regions that are less biased, we do not find that R correlates with the galactocentric distance, velocity dispersion of the gas, or the star formation rate (SFR) measured from 24 um intensity for galaxies in the STING sample. For the EDGE sample, with available IFU optical spectroscopic data at the same resolution, we investigate the relation between R and local stellar and ionized gas properties, such as the instantaneous SFR from Ha emission, gas phase metallicity derived from optical strong line diagnostics, and star formation histories resulting from spectral modeling. For the 12CO-bright regions, we find that in isolated galaxies, R slightly decreases with stellar mass surface density and increases with gas metallicity. On the other hand, there is no significant correlation between R and line width, stellar metallicity, star formation history, SFR, or dust extinction. We also obtain azimuthally averaged R of our sample by stacking multiple 13CO and 12CO spectra in each radial bin. Roughly half of the galaxies show increased stacked R with galactocentric radius, suggesting that a greater fraction of diffuse or less abundant gas contributes to the molecular gas at large radii in general. We conclude that changes in isotopic abundance due to chemical processes or nucleosynthesis do not play a dominant role in R variations on kpc scales. The resolved R is also insensitive to temperature and velocity dispersion. A changing gas density that impacts the opacity and R could account for the general trends we observe on kpc scales. Detailed diagnostics of the properties of molecular ISM will be provided by future high resolution, multi-line molecular observations