Molecular line emission in NGC 1068 imaged with ALMA

II. The chemistry of the dense molecular gas^⋆

¹ Department of Physics and Astronomy, UCL, Gower St., London, WC1E 6BT, UK
e-mail: sv@star.ucl.ac.uk
² Observatorio Astronomico Nacional (OAN) – Observatorio de Madrid, Alfonso XII, 3, 2814 Madrid, Spain
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hg¨el 69, 53121 Bonn, Germany
⁵ Astronomy Department, King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁷ Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 St. Martin d’ Hères, France
⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁹ Department of Radio and Space Science with Onsala Observatory, Chalmers University of Technology, 439 94 Onsala, Sweden
¹⁰ Observatoire de Paris, LERMA, 61 Av. de l’Observatoire, 75014 Paris, France
¹¹ Max-Planck-Institut für Astronomie, Königstuhl, 17, 69117 Heidelberg, Germany
¹² INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, via Gobetti 101, 40129 Bologna, Italy
¹³ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n Granada, Granada 18080, Spain
¹⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherland
¹⁵ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
¹⁶ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
¹⁷ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

Received: 2 May 2014
Accepted: 12 July 2014

Abstract

Aims. We present a detailed analysis of Atacama Large Millimeter/submillimeter Array (ALMA) Bands 7 and 9 data of CO, HCO⁺, HCN, and CS, augmented with Plateau de Bure Interferometer (PdBI) data of the ~200 pc circumnuclear disc (CND) and the ~1.3 kpc starburst ring (SB ring) of NGC 1068, a nearby (D = 14 Mpc) Seyfert 2 barred galaxy. We aim to determine the physical characteristics of the dense gas present in the CND, and to establish whether the different line intensity ratios we find within the CND, as well as between the CND and the SB ring, are due to excitation effects (gas density and temperature differences) or to a different chemistry.

Methods. We estimate the column densities of each species in local thermodynamic equilibrium (LTE). We then compute large one-dimensional, non-LTE radiative transfer grids (using RADEX) by using only the CO transitions first, and then all the available molecules to constrain the densities, temperatures, and column densities within the CND. We finally present a preliminary set of chemical models to determine the origin of the gas.

Results. We find that, in general, the gas in the CND is very dense (>10⁵ cm^-3) and hot (T> 150 K), with differences especially in the temperature across the CND. The AGN position has the lowest CO/HCO⁺, CO/HCN, and CO/CS column density ratios. The RADEX analyses seem to indicate that there is chemical differentiation across the CND. We also find differences between the chemistry of the SB ring and some regions of the CND; the SB ring is also much colder and less dense than the CND. Chemical modelling does not succeed in reproducing all the molecular ratios with one model per region, suggesting the presence of multi-gas phase components.

Conclusions. The LTE, RADEX, and chemical analyses all indicate that more than one gas-phase component is necessary to uniquely fit all the available molecular ratios within the CND. A higher number of molecular transitions at the ALMA resolution is necessary to determine quantitatively the physical and chemical characteristics of these components.