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 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.