Black hole feeding and star formation in NGC 1808^⋆

¹ Observatoire de Paris, LERMA, PSL University, Sorbonne University, CNRS, Paris, France
² National Observatory of Athens (NOA), Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS), 152 36 Athens, Greece
³ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
e-mail: anelise.audibert@iac.es
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁵ Collège de France, 11 Pl. Marcelin Berthelot, 75231 Paris, France
⁶ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy
⁸ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
¹⁰ INAF – Istituto di Radioastronomia, Via Piero Gobetti 101, 40129 Bologna, Italy
¹¹ CNRS, IRAP, 9 Av. Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
¹² IRAM, 300 Rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d’Hères, France
¹³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁴ Dep. of Physics and Astronomy, UCL, Gower Place, London WC1E 6BT, UK
¹⁵ Dep. of Astrophysics, Astronomy & Mechanics, Faculty of Physics, National and Kapodistrian University of Athens, Panepistimiopolis Zografou 15784, Greece
¹⁶ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Penteli, 15236 Athens, Greece
¹⁷ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
¹⁸ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile

Received: 10 November 2020
Accepted: 19 August 2021

Abstract

We report on Atacama Large Millimeter Array (ALMA) observations of CO(3−2) emission in the Seyfert2/starburst galaxy NGC 1808, at a spatial resolution of 4 pc. Our aim is to investigate the morphology and dynamics of the gas inside the central 0.5 kpc and to probe the nuclear feeding and feedback phenomena. We discovered a nuclear spiral of radius 1″ = 45 pc. Within it, we found a decoupled circumnuclear disk or molecular torus of a radius of 0.13″ = 6 pc. The HCN(4−3) and HCO⁺(4−3) and CS(7−6) dense gas line tracers were simultaneously mapped and detected in the nuclear spiral and they present the same misalignment in the molecular torus. At the nucleus, the HCN/HCO⁺ and HCN/CS ratios indicate the presence of an active galactic nucleus (AGN). The molecular gas shows regular rotation, within a radius of 400 pc, except for the misaligned disk inside the nuclear spiral arms. The computations of the torques exerted on the gas by the barred stellar potential reveal that the gas within a radius of 100 pc is feeding the nucleus on a timescale of five rotations or on an average timescale of ∼60 Myr. Some non-circular motions are observed towards the center, corresponding to the nuclear spiral arms. We cannot rule out that small extra kinematic perturbations could be interpreted as a weak outflow attributed to AGN feedback. The molecular outflow detected at ≥250 pc in the NE direction is likely due to supernovae feedback and it is connected to the kpc-scale superwind.