The chemical footprint of AGN feedback in the outflowing circumnuclear disk of NGC 1068

Context. In the nearby (D = 14 Mpc) AGN-starburst composite galaxy NGC 1068, it has been found that the molecular gas in the circumnuclear disk (CND) is outflowing, which is a manifestation of ongoing AGN feedback. The outflowing gas has a large spread of velocities, which likely drive different shock chemistry signatures at different locations in the CND. Aims. We performed a multiline molecular study using two shock tracers, SiO and HNCO, with the aim of determining the gas properties traced by these two species, and we explore the possibility of reconstructing the shock history in the CND. Methods. Five SiO transitions and three HNCO transitions were imaged at high resolution 0.​​″5 − 0.​​″8 with the Atacama Large Millimeter/submillimeter Array (ALMA). We performed both LTE and non-LTE radiative transfer analysis coupled with Bayesian inference process in order to characterize the gas properties, such as the molecular gas density and gas temperature. Results. We found clear evidence of chemical differentiation between SiO and HNCO, with the SiO/HNCO ratio ranging from greater than one on the east of CND to lower than 1 on the west side. The non-LTE radiative transfer analysis coupled with Bayesian inference confirms that the gas traced by SiO has different densities – and possibly temperatures – than that traced by HNCO. We find that SiO traces gas affected by fast shocks while the gas traced by HNCO is either affected by slow shocks or not shocked at all. Conclusions. A distinct differentiation between SiO and HNCO has been revealed in our observations and our further analysis of the gas properties traced by both species confirms the results of previous chemical modelings.