PdBI U/LIRG Survey (PULS): Dense molecular gas in Arp 220 and NGC 6240
1 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
2 Institut de Radio Astronomie Millimetrique, Domaine Universitaire, 38406 St.-Martin d’Hères, France
Received: 25 November 2016
Accepted: 8 April 2017
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 Jk,k′ = 50,4 − 40,4, 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′′ (~700 pc) 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 (~40 K), moderately dense (~103.4 cm-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.
Key words: galaxies: abundances / galaxies: ISM / ISM: molecules / galaxies: starburst
© ESO, 2017