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We report the results of HCN(J=4-3) and HCO+(J=4-3) observations of twoluminous infrared galaxies (LIRGs), NGC 4418 and Arp 220, made using theAtacama Submillimeter Telescope Experiment (ASTE). The ASTE wide-bandcorrelator provided simultaneous observations of HCN(4-3) and HCO+(4-3) lines,and a precise determination of their flux ratios. Both galaxies showed highHCN(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 tothose expected for fully thermalized (sub-thermally excited) gas in bothsources, in spite of HCN's higher critical density. If we assume collisionalexcitation and neglect an infrared radiative pumping process, our non-LTEanalysis suggests that HCN traces gas with significantly higher density thanHCO+. In Arp 220, we separated the double-peaked HCN(4-3) emission into theeastern and western nuclei, based on velocity information. We confirmed thatthe eastern nucleus showed a higher HCN(4-3) to HCN(1-0) flux ratio, and thuscontained a larger amount of highly excited molecular gas than the westernnucleus.

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