A λ = 3 mm molecular line survey of NGC 1068

Chemical signatures of an AGN environment ^⋆,⋆⋆

¹ European Southern Observatory, Avda. Alonso de Córdova 3107, Vitacura, Santiago, Chile
e-mail: raladro@eso.org
² University College London (UCL), Dept. of Physics & Astronomy, Gower Street, London WC1E 6BT, UK
³ Sub-Department of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Joint ALMA Observatory, Avda. Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁶ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁷ Instituto de Radioastronomía Milimétrica, Avda. Divina Pastora, 7, Local 20, 18012 Granada, Spain

Received: 30 July 2012
Accepted: 12 October 2012

Abstract

Aims. We study the molecular composition of the interstellar medium (ISM) surrounding an active galactic nucleus (AGN), by making an inventory of molecular species and their abundances, to establish a chemical differentiation between starburst galaxies and AGN.

Methods. We used the IRAM-30 m telescope to observe the central 1.5–2 kpc region of NGC 1068, covering the frequencies between 86.2 GHz and 115.6 GHz. Using Boltzmann diagrams, we calculated the column densities of the detected molecules. We used a chemical model to reproduce the abundances found in the AGN, to determine the origin of each detected species, and to test the influence of UV fields, cosmic rays, and shocks on the ISM.

Results. We identified 24 different molecular species and isotopologues, among which HC₃N, SO, N₂H⁺, CH₃CN, NS, ¹³CN, and HN¹³C are detected for the first time in NGC 1068. A comparison of the abundances in the nuclear regions of NGC 1068, M 82, and NGC 253 allowed us to establish a chemical differentiation between starburst galaxies and AGN. Two abundant species in starburst galaxies, H₂CO and CH₃CCH, are not detected in NGC 1068, probably because they are destroyed by UV fields or shocks. On the other hand, species such as CN, SiO, HCO⁺, and HCN, are enhanced by cosmic ray radiation fields. We obtained the upper limits to the isotopic ratios ¹²C/¹³C = 49, ¹⁶O/¹⁸O = 177, and ³²S/³⁴S = 5. These ratios are much lower in this AGN than in starburst galaxies. Our chemical models suggest that the chemistry in the nucleus of NGC 1068 is strongly influenced by cosmic rays, although high values of both cosmic rays and far ultraviolet (FUV) radiation fields also explain the observations well. C-shocks can explain the abundances of C₂H and H₂CO, but do not strongly affect the abundances of the other detected species.

Conclusions. The gas in the nucleus of NGC 1068 has a different chemical composition than starburst galaxies. The distinct physical processes dominating galaxy nuclei (e.g. C-shocks, UV fields, X-rays, cosmic rays) leave clear imprints in the chemistry of the gas, which allow the nucleus activity to be characterised by its molecular abundances.