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This paper is part of a multi-species survey of line emission from themolecular gas in the circum-nuclear disk (CND) of the Seyfert 2 galaxy NGC1068.Single-dish observations have provided evidence that the abundance of siliconmonoxide(SiO) in the CND of NGC1068 is enhanced by 3-4 orders of magnitude withrespect to the values typically measured in quiescent molecular gas in theGalaxy. We aim at unveiling the mechanism(s) underlying the SiO enhancement. Wehave imaged with the IRAM Plateau de Bure interferometer the emission of theSiO(2-1) and CN(2--1) lines in NGC1068 at 150pc and 60pc spatial resolution,respectively. We have also obtained complementary IRAM 30m observations of HNCOand methanol (CH3OH) lines. SiO is detected in a disk of 400pc size around theAGN. SiO abundances in the CND of (1-5)xE-09 are about 1-2 orders of magnitudeabove those measured in the starburst ring. The overall abundance of CN in theCND is high: (0.2-1)xE-07. The abundances of SiO and CN are enhanced at theextreme velocities of gas associated with non-circular motions close to the AGN(r<70pc). Abundances measured for CN and SiO, and the correlation of CN/CO andSiO/CO ratios with hard X-ray irradiation, suggest that the CND of NGC1068 hasbecome a giant X-ray dominated region (XDR). The extreme properties ofmolecular gas in the circum-nuclear molecular disk of NGC1068 result from theinterplay between different processes directly linked to nuclear activity.Whereas XDR chemistry offers a simple explanation for CN and SiO in NGC1068,the relevance of shocks deserves further scrutiny. The inclusion of dust grainchemistry would help solve the controversy regarding the abundances of othermolecular species, like HCN, which are under-predicted by XDR models.

Molecular gas chemistry in AGN. II. High-resolution imaging of SiO emission in NGC1068: shocks or XDR?