{
  "id": "1507.07565",
  "version": "v1",
  "published": "2015-07-27T20:00:58.000Z",
  "updated": "2015-07-27T20:00:58.000Z",
  "title": "Modelling the Molecular Gas in NGC 6240",
  "authors": [
    "R. Tunnard",
    "T. R. Greve",
    "S. Garcia-Burillo",
    "J. Graciá Carpio",
    "A. Fuente",
    "L. Tacconi",
    "R. Neri",
    "A. Usero"
  ],
  "comment": "37 pages, 30 figures, 10 tables. Submitted to ApJ. Comments welcome",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "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.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2015-07-27T20:00:58.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "dense clouds",
      "phase model",
      "markov chain monte carlo",
      "dense gas",
      "derive posterior probability density functions"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 37,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}