ASTE Simultaneous HCN (4-3) and HCO+(4-3) Observations of the Two Luminous Infrared Galaxies NGC 4418 and Arp 220

We report the results of HCN (J = 4-3) and HCO+(J = 4-3) observations of two luminous infrared galaxies, NGC 4418 and Arp 220, made using the Atacama Submillimeter Telescope Experiment (ASTE). The ASTE wide-band correlator provided simultaneous observations of HCN (4-3) and HCO+(4-3) lines, and a precise determination of their flux ratios. Both galaxies showed high HCN (4-3) to HCO+(4-3) flux ratios of > 2, possibly due to AGN-related phenomena. The J = 4-3 to J = 1-0 transition flux ratios for HCN (HCO+) are similar to those expected for fully thermalized (sub-thermally excited) gas in both sources, in spite of HCN's higher critical density. If we assume collisional excitation and neglect an infrared radiative pumping process, our non-LTE analysis suggests that HCN traces gas with significantly higher density than HCO+. In Arp 220, we separated the double-peaked HCN (4-3) emission into eastern and western nuclei, based on velocity information. We confirmed that the eastern nucleus showed a higher HCN (4-3) to HCN (1-0) flux ratio, and thus contained a larger amount of highly excited molecular gas than the western nucleus.