2010/01/04 by Masatoshi Imanishi, Kouichiro Nakanishi, Masako Yamada +2
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Excitation #Excited state #Flux (metallurgy) #Galaxies: Formation, Evolution, Phenomena #Galaxy #Infrared #Luminous infrared galaxy #Radiative transfer #Star formation #astro-ph.CO
paper · pdf · doi:10.1093/pasj/62.1.201
21 pages, 4 figures, accepted for publication in PASJ (Vol.62, No.1, 2010 Feb 25 issue)
arxiv created 2010/01/04 · openalex publication_date 2010/02/25 · arxiv updated 2015/05/14 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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.