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

Abstract

Abstract 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.