Title: PdBI U/LIRG Survey (PULS): Dense Molecular Gas in Arp 220 and NGC 6240,
Abstract: Aims. We present new IRAM Plateau de Bure Interferometer observations of Arp
220 in HCN, HCO$^{+}$, HN$^{13}$C J=1-0, C$_{2}$H N=1-0, SiO J = 2-1, HNCO
J$_{k,k'}$ = 5$_{0,4}$ - 4$_{0,4}$, CH$_{3}$CN(6-5), CS J=2-1 and 5-4 and
$^{13}$CO J=1-0 and 2-1 and of NGC 6240 in HCN, HCO$^{+}$ J = 1-0 and C$_{2}$H
N = 1-0. In addition, we present Atacama Large Millimeter/submillmeter Array
science verification observations of Arp 220 in CS J = 4-3 and
CH$_{3}$CN(10-9). Various lines are used to analyse the physical conditions of
the molecular gas including the [$^{12}$CO]/[$^{13}$CO] and
[$^{12}$CO]/[C$^{18}$O] 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 $^{12}$CO, $^{13}$CO and C$^{18}$O lines of
Arp 220 at ~2" (~700 pc) scales, where the $^{12}$CO and C$^{18}$O measurements
were obtained from literature. Results. Line ratios of optically thick lines
such as $^{12}$CO show smoothly varying ratios while the line ratios of
optically thin lines such as $^{13}$CO 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
(~10$^{3.4}$ cm$^{-3}$) molecular gas component averaged over the two nuclei.
We find [$^{12}$CO]/[$^{13}$CO] and [$^{12}$CO]/[C$^{18}$O] abundance ratios of
~90 for both. The abundance enhancement of C$^{18}$O can be explained by
stellar nucleosynthesis enrichment of the interstellar medium.