2017A&A...604A...2S

Aims. We present new IRAM Plateau de Bure Interferometer observations of Arp 220 in HCN, HCO⁺, HN¹³C J=1-0, C₂H N=1-0, SiO J=2-1, HNCO J_k,k'=5_0,4-4_0,4, CH₃CN(6-5), CS J=2-1 and 5-4 and ¹³CO J=1-0 and 2-1 and of NGC 6240 in HCN, HCO⁺ J=1-0 and C₂H N=1-0. In addition, we present Atacama Large Millimeter/submill-meter Array science verification observations of Arp 220 in CS J=4-3 and CH₃CN(10-9). Various lines are used to analyse the physical conditions of the molecular gas including the [¹²CO]/[¹³CO] and [¹²CO]/[C¹⁸O] abundance ratios. These observations will be made available to the public.
Methods. We create brightness temperature line ratio maps to present the different physical conditions across Arp 220 and NGC 6240. In addition, we use the radiative transfer code RADEX and a Monte Carlo Markov chain likelihood code to model the ¹²CO, ¹³CO and C¹⁸O lines of Arp 220 at ∼2'' (∼700pc) scales, where the ¹²CO and C¹⁸O measurements were obtained from literature.
Results. Line ratios of optically thick lines such as ¹²CO show smoothly varying ratios while the line ratios of optically thin lines such as ¹³CO show a east-west gradient across Arp 220. The HCN/HCO⁺ line ratio differs between Arp 220 and NGC 6240, where Arp 220 has line ratios above 2 and NGC 6240 below 1. The radiative transfer analysis solution is consistent with a warm (∼40K), moderately dense (∼10^3.4cm^–3) molecular gas component averaged over the two nuclei. We find [¹²CO]/[¹³CO] and [¹²CO]/[C¹⁸O] abundance ratios of ∼90 for both. The abundance enhancement of C¹⁸O can be explained by stellar nucleosynthesis enrichment of the interstellar medium.