Stored in the archives: Uncovering the CN/CO intensity ratio with ALMA in nearby U/LIRGs

Abstract

We present an archival Atacama Large Millimeter/submillimeter Array (ALMA) study of the CN N = 1 - 0 / CO J = 1 - 0 intensity ratio in nearby (z < 0.05) Ultra Luminous and Luminous Infrared Galaxies (U/LIRGs). We identify sixteen U/LIRGs that have been observed in both CN and CO lines at $\sim$ 500 pc resolution based on sixteen different ALMA projects. We measure the (CN bright)/CO and (CN bright)/(CN faint) intensity ratios at an ensemble of molecular clouds scales (CN bright = CN N = 1 - 0, J = 3/2 - 1/2; CN faint = CN N = 1 - 0, J = 1/2 - 1/2 hyperfine groupings). Our global measured (CN bright)/CO ratios range from 0.02-0.15 in LIRGs and 0.08-0.17 in ULIRGs. We attribute the larger spread in LIRGs to the variety of galaxy environments included in our sample. Overall, we find that the (CN bright)/CO ratio is higher in nuclear regions, where the physical and excitation conditions favour increased CN emission relative to the disk regions. 10 out of 11 galaxies which contain well-documented active galactic nuclei show higher ratios in the nucleus compared to the disk. Finally, we measure the median resolved (CN bright)/(CN faint) ratio and use it to estimate the total integrated CN line optical depth in ULIRGs ($\tau \sim$ 0.96) and LIRGs ($\tau \sim$ 0.23). The optical depth difference is likely due to the higher molecular gas surface densities found in the more compact ULIRG systems.