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arXiv · 1001.0582

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

Masatoshi Imanishi·Kouichiro Nakanishi·Masako Yamada·Yoichi Tamura·Kotaro Kohno
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Abstract

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.

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Masatoshi Imanishi, Kouichiro Nakanishi, Masako Yamada, Yoichi Tamura, Kotaro Kohno. 2010-01-04. ASTE Simultaneous HCN(4-3) and HCO+(4-3) Observations of the Two Luminous Infrared Galaxies NGC 4418 and Arp 220. https://doi.org/10.1093/pasj%2F62.1.201

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