Modelling the Molecular Gas in NGC 6240

R. Tunnard, T. R. Greve, S. Garcia-Burillo, J. Graciá Carpio, A. Fuente, L. Tacconi, R. Neri, A. Usero

Published 2015-07-27Version 1

We present the first observations of H$^{13}$CN$(1-0)$, H$^{13}$CO$^+(1-0)$ and SiO$(2-1)$ in NGC 6240, obtained with the IRAM PdBI. Combining a Markov Chain Monte Carlo (MCMC) code with Large Velocity Gradient (LVG) modelling we derive posterior probability density functions (pdfs) for the dense gas parameters, including mass$-$luminosity conversion factors, finding a large amount of dense molecular gas $(\sim10^{10}M_\odot)$ in cold, dense clouds ($T_k\sim10$ K, $n_{{\rm H}_2}\sim10^6$ cm$^{-3}$) with a small volume filling factor $(<0.002)$. Including literature CO data we present simultaneously fitted multi-species, two phase models which spontaneously separate into a hot, diffuse phase ($\log_{10}\left(T_k / [{\rm K}]\right) = 3.2^{3.3}_{3.1}$, $\log_{10}\left(n_{{\rm H}_2} / [{\rm cm}^{-3}]\right)=3.6^{3.8}_{3.5}$) and a cold, dense phase ($\log_{10}\left(T_k / [{\rm K}]\right) = 0.9^{0.9}_{0.8}$, $\log_{10}\left(n_{{\rm H}_2} / [{\rm cm}^{-3}]\right)=6.6^{6.8}_{6.3}$). A restricted three phase model is used to include the ubiquitous diffuse, CO bearing gas phase and we derive a global $\alpha_{\rm CO}=1.5^{7.1}_{1.1}$ with gas masses $\log_{10}\left(M / [M_\odot]\right)=10.1_{10.0}^{10.8}$, dominated by the dense gas. We find that the [$^{12}$C]/[$^{13}$C] ratio is only slightly elevated ($98^{230}_{65}$), contrary to the very high [CO]/[$^{13}$CO] ratio (300-500) reported in the literature. The high [HCN]/[H$^{13}$CN] and [HCO$^+$]/[H$^{13}$CO$^+$] abundance ratios $(300^{500}_{200})$ we find are due to isotope fractionation in the cold, dense clouds.