2024MNRAS.527.2963L

We present an archival Atacama Large Millimeter/submillimeter Array (ALMA) study of the cyanide radical (CN) N = 1 - 0/carbon monoxide (CO) J = 1 - 0 intensity ratio in nearby (z < 0.05) ultra-luminous and luminous infrared galaxies (U/LIRGs). We identify 16 U/LIRGs that have been observed in both CN and CO lines at ∼500 pc resolution based on 16 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 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 disc regions. 10 out of 11 galaxies which contain well-documented active galactic nuclei show higher ratios in the nucleus compared with the disc. 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 (τ ∼ 0.96) and LIRGs (τ ∼ 0.23). The optical depth difference is likely due to the higher molecular gas surface densities found in the more compact ULIRG systems.