ALMA images the many faces of the NGC 1068 torus and its surroundings^⋆

¹ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
e-mail: s.gburillo@oan.es
² Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
³ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁵ Centro de Astrobiología (CSIC-INTA), ESAC Campus, 28692 Villanueva de la Cañada, Madrid, Spain
⁶ INAF-Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁷ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 Place Jules Janssen, 92195 Meudon, France
⁸ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
⁹ European Southern Observatory (ESO), Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
¹⁰ Department of Physics and Astronomy, UCL, Gower Place, London WC1E 6BT, UK
¹¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 Leiden, The Netherlands
¹² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹³ Astronomy Department, King Abdulazizi University, PO Box 80203, Jeddah 21589, Saudi Arabia
¹⁴ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
¹⁵ European Southern Observatory (ESO), Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
¹⁶ Institut de Radio Astronomie Millimétrique (IRAM), 300 Rue de la Piscine, Domaine Universitaire de Grenoble, 38406 St. Martin d’Hères, France
¹⁷ Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

Received: 30 August 2019
Accepted: 13 October 2019

Abstract

Aims. We investigate the fueling and the feedback of nuclear activity in the nearby (D = 14 Mpc) Seyfert 2 barred galaxy NGC 1068 by studying the distribution and kinematics of molecular gas in the torus and its connections to the host galaxy disk.

Methods. We used the Atacama Large Millimeter Array (ALMA ) to image the emission of a set of molecular gas tracers in the circumnuclear disk (CND) and the torus of the galaxy using the CO(2–1), CO(3–2), and HCO⁺(4–3) lines and their underlying continuum emission with high spatial resolutions (0.03″ − 0.09″ ≃ 2 − 6 pc). These transitions, which span a wide range of physical conditions of molecular gas (n(H₂)⊂10³ − 10⁷ cm⁻³), are instrumental in revealing the density radial stratification and the complex kinematics of the gas in the torus and its surroundings.

Results. The ALMA images resolve the CND as an asymmetric ringed disk of D ≃ 400 pc in size and ≃1.4 × 10⁸ M_⊙ in mass. The CND shows a marked deficit of molecular gas in its central ≃130 pc region. The inner edge of the ring is associated with the presence of edge-brightened arcs of NIR polarized emission, which are identified with the current working surface of the ionized wind of the active galactic nucleus (AGN). ALMA proves the existence of an elongated molecular disk/torus in NGC 1068 of M_torus^gas ≃ 3 × 10⁵ M_⊙, which extends over a large range of spatial scales D ≃ 10 − 30 pc around the central engine. The new observations evidence the density radial stratification of the torus: the HCO⁺(4–3) torus, with a full size D_{HCO⁺(4 − 3)} = 11 ± 0.6 pc, is a factor of between two and three smaller than its CO(2–1) and CO(3–2) counterparts, which have full sizes of D_{CO(3 − 2)} = 26 ± 0.6 pc and D_{CO(2 − 1)} = 28 ± 0.6 pc, respectively. This result brings into light the many faces of the molecular torus. The torus is connected to the CND through a network of molecular gas streamers detected inside the CND ring. The kinematics of molecular gas show strong departures from circular motions in the torus, the gas streamers, and the CND ring. These velocity field distortions are interconnected and are part of a 3D outflow that reflects the effects of AGN feedback on the kinematics of molecular gas across a wide range of spatial scales around the central engine. In particular, we estimate through modeling that a significant fraction of the gas inside the torus (≃ 0.4 − 0.6 × M_torus^gas) and a comparable amount of mass along the gas streamers are outflowing. However, the bulk of the mass, momentum, and energy of the molecular outflow of NGC 1068 is contained at larger radii in the CND region, where the AGN wind and the radio jet are currently pushing the gas assembled at the Inner Lindblad Resonance (ILR) ring of the nuclear stellar bar.

Conclusions. In our favored scenario a wide-angle AGN wind launched from the accretion disk of NGC1068 is currently impacting a sizable fraction of the gas inside the torus. However, a large gas reservoir (≃1.2 − 1.8 × 10⁵ M_⊙), which lies close to the equatorial plane of the torus, remains unaffected by the feedback of the AGN wind and can therefore continue fueling the AGN for at least ≃1 − 4 Myr. Nevertheless, AGN fueling currently seems thwarted on intermediate scales (15 pc ≤r ≤ 50 pc).