Abstract
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 CH3OH with the aim of characterizing the gas properties probed by CH3OH in the CND of NGC 1068 and investigating its potential association with molecular shocks.
Methods. Multi-transition CH3OH 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 CH3OH point to ∼106 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 CH3OH suggests shocks and subsequent fast cooling as the origin of the observed gas-phase CH3OH 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 CH3OH exclusively traces slow and non-dissociative shocks, or whether the CH3OH abundance can actually be boosted in both fast and slow shocks.