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Published February 1, 2017 | Published
Journal Article Open

What Lurks in ULIRGs?—Probing the Chemistry and Excitation of Molecular Gas in the Nuclei of Arp 220 and NGC 6240


We have imaged the dense star-forming regions of Arp 220 and NGC 6240 in the 3 mm band transitions of CO, HCN, HCO^+, HNC, and CS at 0farcs5–0farcs8 resolution using CARMA. Our data set images all these lines at similar resolutions and high sensitivity, and can be used to derive line ratios of faint high excitation lines. In both the nuclei of Arp 220, the HCN/HNC ratios suggest chemistry of X-ray Dominated Regions (XDRs)—a likely signature of an active galactic nucleus. In NGC 6240, there is no evidence of XDR type chemistry, but there the bulk of the molecular gas is concentrated between the nuclei rather than on them. We calculated molecular H_2 densities from excitation analysis of each of the molecular species. It appears that the abundances of HNC and HCO^+ in Ultra Luminous Infrared Galaxies may be significantly different from those in galactic molecular clouds. The derived H_2 volume densities are ~5 × 10^4 cm^(−3) in the Arp 220 nuclei and ~10^4 cm^(−3) in NGC 6240.

Additional Information

© 2017. The American Astronomical Society. Received 2016 February 23; revised 2016 November 25; accepted 2016 December 7; published 2017 January 23. We thank our referee for the thorough report which has helped improve the paper. We gratefully acknowledge the assistance of Anneila Sargent for suggestions and improvements on the manuscript. We thank the staff of CARMA for their help with carrying out the observations and calibrations. We thank Marc W. Pound and Peter Teuben for their invaluable help with data reduction. CARMA development and operations were supported by the National Science Foundation grants AST-0838260 and AST-1140063 under a cooperative agreement, and by the CARMA partner universities. We acknowledge student support from NRAO under grants AST-0956545 and AST-0836064. We thank Jin Koda for lending us the LVG modeling code.

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