A λ = 3 mm molecular line survey of NGC 1068 - Chemical signatures of an AGN environment

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 + ,C H 3CN, NS, 13 CN, and HN 13 C 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 12 C/ 13 C = 49, 16 O/ 18 O = 177, and 32 S/ 34 S = 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 C2 Ha nd H 2CO, 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.