Molecular line emission in NGC 1068 imaged with ALMA II. The chemistry of the dense molecular gas
Journal article, 2014
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.
galaxies: active / galaxies: individual: NGC 1068 / galaxies: ISM / galaxies: nuclei / molecular processes / radio lines: galaxies
Author
S. Viti
UCL
S. G. Burillo
A. Fuente
L. K. Hunt
Osservatorio Astrofisico Di Arcetri
A. Usero
C. Henkel
King Abdulaziz University
Max Planck Institut fur Radioastronomie
A. Eckart
Max Planck Institut fur Radioastronomie
Universitat zu Koln
S. Martin
Universite Grenoble Alpes
M. Spaans
University of Groningen, Kapteyn Astronomical Institute
Sebastien Muller
Chalmers, Earth and Space Sciences, Onsala Space Observatory
F. Combes
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
M. Krips
Universite Grenoble Alpes
E. Schinnerer
Max Planck Institut fur Astronomie
V. Casasola
INAF Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna
Francesco Costagliola
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
I. Marquez
CSIC - Instituto de Astrofísica de Andalucía (IAA)
P. Planesas
P.P. van der Werf
Leiden Observatory Research Institute
Susanne Aalto
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
A. J. Baker
Rutgers, The State University of New Jersey
F. Boone
Universite de Toulouse
L. J. Tacconi
Max Planck Institut fur extraterrestrische Physik
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 570 28- A28Subject Categories (SSIF 2011)
Astronomy, Astrophysics and Cosmology
Infrastructure
Onsala Space Observatory
DOI
10.1051/0004-6361/201424116