ALMA observations of molecular tori around massive black holes^⋆

¹ Observatoire de Paris, LERMA, CNRS, PSL Univ., Sorbonne University, UPMC, Paris, France
e-mail: francoise.combes@obspm.fr
² College de France, 11 Pl. Marcelin Berthelot, 75231 Paris, France
³ Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
⁵ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁶ Dep. of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁷ INAF – Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy
⁸ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁹ RAM, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d’Hères, France
¹⁰ Dep. of Physics and Astronomy, UCL, Gower Place, London WC1E 6BT, UK
¹¹ Academia Sinica, Institute of Astronomy and Astrophysics, Taiwan
¹² Dep. of Astrophysics, Astronomy & Mechanics, Faculty of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou 15784, Greece
¹³ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Penteli 15236 Athens, Greece
¹⁴ Leiden Observatory, Leiden Univ., PO Box 9513 2300 RA Leiden, The Netherlands
¹⁵ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
¹⁶ Joint ALMA Observatory, Alonso de Córdova, 3107 Vitacura, Santiago 763-0355, Chile

Received: 2 November 2018
Accepted: 26 January 2019

Abstract

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO(3-2) emission in a sample of seven Seyfert/LINER galaxies at the unprecedented spatial resolution of 0_.^″1 = 4−8 pc. Our aim is to explore the close environment of active galactic nuclei (AGN), and the dynamical structures leading to their fueling, through the morphology and kinematics of the gas inside the sphere of influence of the black hole. The selected galaxies host low-luminosity AGN and have a wide range of activity types (Seyferts 1 to 2, LINERs), and barred or ringed morphologies. The observed maps reveal the existence of circumnuclear disk structures, defined by their morphology and decoupled kinematics, in most of the sample. We call these structures molecular tori, even though they often appear as disks without holes in the center. They have varying orientations along the line of sight, unaligned with the host galaxy orientation. The radius of the tori ranges from 6 to 27 pc, and their mass from 0.7 × 10⁷ to 3.9 × 10⁷ M_⊙. The most edge-on orientations of the torus correspond to obscured Seyferts. In only one case (NGC 1365), the AGN is centered on the central gas hole of the torus. On a larger scale, the gas is always piled up in a few resonant rings 100 pc in scale that play the role of a reservoir to fuel the nucleus. In some cases, a trailing spiral is observed inside the ring, providing evidence for feeding processes. More frequently, the torus and the AGN are slightly off-centered with respect to the bar-resonant ring position, implying that the black hole is wandering by a few 10 pc amplitude around the center of mass of the galaxy. Our spatial resolution allows us to measure gas velocities inside the sphere of influence of the central black holes. By fitting the observations with different simulated cubes, varying the torus inclination and the black hole mass, it is possible to estimate the mass of the central black hole, which is in general difficult for such late-type galaxies, with only a pseudo-bulge. In some cases, AGN feedback is revealed through a molecular outflow, which will be studied in detail in a subsequent article.