ALMA uncovers optically thin and multi-component CO gas in the outflowing circumnuclear disk of NGC 1068

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Transdisciplinary Research Area (TRA Matter) and Argelander Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
³ Department of Physics and Astronomy, UCL, Gower Street, London WC1E 6BT, UK
⁴ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain

^⋆ Corresponding authors: yuzezhang@mail.strw.leidenuniv.nl; viti@strw.leidenuniv.nl; s.gburillo@oan.es

Received: 8 January 2025
Accepted: 2 April 2025

Abstract

Active galactic nuclei (AGNs) influence host galaxies through winds and jets that generate molecular outflows, which are traceable with ¹²CO line emissions using the Atacama Large Millimeter Array (ALMA). Leveraging ALMA observations, recent studies have proposed a 3D outflow geometry in the nearby Seyfert II galaxy NGC 1068–a primary testbed for AGN unification theories. Utilizing ALMA data of CO(2–1), CO(3–2), and CO(6–5) transitions at ∼0.1″ (∼7 pc) resolution, we analyzed temperature, density, and kinematics within the circumnuclear disk (CND) of NGC 1068, focusing on molecular outflows. We selected regions across the CND based on a previously modeled AGN wind bicone. We performed local thermodynamic equilibrium (LTE) analysis to infer column densities and rotational temperatures, which revealed optically thin gas with X_CO factors 4.8±0.4−9.6±0.9 times smaller than the Milky Way value. Consequently, the molecular mass outflow rate within 40 × 40 pc regions across the CND is mostly below 5.5 M_⊙ yr⁻¹, with the majority contributed from the area northeast of the AGN position (α₂₀₀₀ = 02^h42^m40.776^s, δ₂₀₀₀=−00°00′47.714″). After subtracting the rotation curve of the CND, we fit averaged line profiles for each sampled region using single and weighted multi-component Gaussian models to investigate the kinematics of the non-rotating gas. The fitting results show that some line profiles close to or within the AGN wind bicone require multi-component Gaussian models, with each component exhibiting significant velocity departures from the galaxy's mean motion–a hallmark of a multi-component molecular outflow. We observed lateral variations of CO gas kinematics along the edge and center of the AGN wind bicone as well as a misalignment of the orientation and spread between the molecular outflow and the ionized outflow. Overall, due to the optically thin condition, the dynamic impact of the ionized outflow to molecular gas inside the CND might not be as substantial as expected. Regardless, the outflowing molecular gas across the CND exhibits complex kinematics, highlighted by an asymmetry between the northeastern and southern CND, and our analyses do not eliminate the 3D outflow geometry as a possible outflow scenario within the CND of NGC 1068.