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.