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Paper: The SMA Observations of CO Line Emission from Arp 220: Super Clusters of Giant Molecular Clouds in the Merging Nuclei
Volume: 395, Frontiers of Astrophysics: A Celebration of NRAO's 50th Anniversary
Page: 384
Authors: Zhao, J.-H.; An, T.
Abstract: We present the results from observations of Arp 220 in multiple CO transitions of J=6–5, 3–2, 2–1 and the isotope 13CO at J=2–1 at 690, 340 and 230 GHz with the Submillimeter Array (SMA) in angular resolutions of sub-arcsec to a few arcsec. Four regions (Arp 220S, Arp220W, Arp220NE, Arp220E) of concentration in CO J=6–5 line emission have been observed in the central 2 arcsec. The typical line intensity ratios of CO J=6–5/CO J=3–2 ∼ 1.5 and CO J=2–1/13CO J=2–1 ∼ 15 are determined in the nuclear region. Based on the measured ratios, we modeled the excitation condition with the large-velocity-gradient (LVG) approximation. The LVG-model fitting suggests that the CO J=6–5 emission arises from dense (n(H2) ∼ 105cm−3) and warm (Tk ∼ 100 K) cloud components. Each of the four CO J=6–5 clumps contains at least a few times 108 M molecular gas, suggesting presence of a few superclusters of giant molecular clouds (GMCs) in Arp 220.The kinematics observed from CO J=3–2 in sub-arcsec resolution show evidence for association of a mini-rotating disk or a ring with each of the superclusters of GMCs. Two of them, Arp 220W and Arp 220E, are associated with the two well-known nuclear continuum sources. No significant radio and submillimeter continuum emission has been detected from the other two, Arp 220S and Arp220NE, suggesting that they are in a primeval stage of star formation. Two molecular tails are also observed in a larger scale, suggesting that they might be of tidal origin. Based on the SMA observations along with the previous observations by many other groups, the nuclear starburst in Arp 220 is probably triggered by the tidal forces of the merging nuclei. Properties determined for the superclusters of GMCs suggest that they are in different stages of star formation.
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