The Relationship Between 13CO/C18O and Star Formation Rate Surface Density

Optically thin ¹³CO and C¹⁸O allow us to investigate the physical causes for abundance variations across the spiral disks of galaxies. Correlation between the ¹³CO/C¹⁸O ratio and Σ_SFR can indicate either chemical fractionation or selective enrichment by stellar nucleosynthesis. We use data taken by the Green Bank Telescope (GBT) and Argus to measure the ¹³CO/C¹⁸O ratio and a combination of infrared (IR) and far ultraviolet to trace star formation rate surface density (Σ_SFR) in nearby spiral galaxies. This project uses three different region selection methods to find the relation: the central 15” of each galaxy, 0.16×r₂₅ of each galaxy, and radial profiles starting from the center and increasing by 15” galactocentric radius for each region. After plotting the ¹³CO/C¹⁸O ratio as a function of Σ_SFR for all region selection methods, we found that the ¹³CO/C¹⁸O ratio increased with increasing Σ_SFR with a slope of 0.07±0.03 for central regions, decreased with increasing Σ_SFR with a slope of -0.7±1.0 for whole galaxy regions, and consistently decreased for the radial profiles with two galaxies (NGC 3631 and NGC 5055) as outliers. We see evidence for both chemical fractionation and selective enrichment, indicating that no one mechanism appears to dominate in all scenarios.