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 (LIRGs), 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 the
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.
