Multi-transition study of methanol towards NGC 1068 with ALMA

¹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
e-mail: kyhuang@strw.leidenuniv.nl
² Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain

Received: 29 November 2023
Accepted: 11 April 2024

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.