Multi-transition study of methanol towards NGC 1068 with ALMA

Context. The outflowing molecular gas in the circumnuclear disc (CND) of the nearby ( D = 14 Mpc) AGN-starburst composite galaxy NGC 1068 is considered as a manifestation of ongoing AGN feedback. The large spread of velocities from the outflowing gas is likely driving various kinds of shock chemistry across the CND. Aims. We performed a multiline molecular study using CH 3 OH with the aim of characterizing the gas properties probed by CH 3 OH in the CND of NGC 1068 and investigating its potential association with molecular shocks. Methods. Multi-transition CH 3 OH were imaged at the resolution of 0.″5 − 0.″8 with the Atacama Large Millimeter/submillimeter Array (ALMA). We performed a non-LTE radiative transfer analysis coupled with a Bayesian inference process in order to determine the gas properties such as the gas volume density and the gas kinetic temperature. Results. The gas densities traced by CH 3 OH point to ∼10 6 cm –3 across all the CND regions. The gas kinetic temperature cannot be well constrained in any of the CND regions, though the inferred temperature is likely low (≲100 K). Conclusions. The low gas temperature traced by CH 3 OH suggests shocks and subsequent fast cooling as the origin of the observed gas-phase CH 3 OH abundance. We also note that the E-/A-isomer column-density ratio inferred is fairly close to unity, which is, interestingly, different from the Galactic measurements in the literature. It remains inconclusive whether CH 3 OH exclusively traces slow and non-dissociative shocks, or whether the CH 3 OH abundance can actually be boosted in both fast and slow shocks.