Dense-gas properties in Arp 220 revealed by isotopologue lines

Abstract

We present observations of isotopologue lines of dense-gas tracers at 3 mm and 1 mm towards the nearest ultra-luminous infrared galaxy Arp 220. The 3-mm and 1-mm observations were performed with the Institut de Radioastronomie Millimetrique 30-m telescope and the Atacama Pathfinder Experiment 12-m telescope, respectively. We detected (HCN)-C-13 and (HNC)-C-13 in 1-0 and 3-2, and (HCN)-N-15 1-0, among which (HCN)-N-15 1-0 and (HNC)-C-13 1-0 are detected in Arp 220 for the first time. The (HCO+)-C-13 1-0 and 3-2 lines are unlikely to be detected because of the confusion of SiO lines. We find that the ratio of the line brightness temperatures of (HNC)-C-13 3-2/1-0 is 2.4, which is significantly higher than that of (HCN)-C-13 3-2/1-0 (0.73). This indicates that (HNC)-C-13 and HNC molecules are in denser regions than (HCN)-C-13 and HCN molecules. With the line ratio of (HCN)-C-13 1-0 and (HCN)-N-15 1-0, the N-14/N-15 ratio was estimated to be 440(-82)(+140), which is larger than that of the local interstellar medium.