ALMA reveals the feeding of the Seyfert 1 nucleus in NGC 1566^⋆

¹ Observatoire de Paris, LERMA, CNRS UMR8112, 61 Av. de l’Observatoire, 75014 Paris, France
e-mail: francoise.combes@obspm.fr
² Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014- Madrid, Spain
³ INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, via Gobetti 101, 40129 Bologna, Italy
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint Martin d’Hères, France
⁶ Dep. of Physics & Astronomy, Rutgers, the State University of New Jersey, 136 Frelinghuysen road, Piscataway NJ 08854, USA
⁷ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁸ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁹ Instituto de Astrofísica de Andalucía (CSIC), Apdo 3004, 18080 Granada, Spain
¹⁰ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching bei München, Germany

Received: 15 January 2014
Accepted: 10 March 2014

Abstract

We report ALMA observations of CO(3–2) emission in the Seyfert 1 galaxy NGC 1566, at a spatial resolution of 25 pc. Our aim is to investigate the morphology and dynamics of the gas inside the central kpc, and to probe nuclear fueling and feedback phenomena. NGC 1566 has a nuclear bar of 1.7 kpc radius and a conspicuous grand design spiral starting from this radius. The ALMA field of view, of diameter 0.9 kpc, lies well inside the nuclear bar and reveals a molecular trailing spiral structure from 50 to 300 pc in size, which is contributing to fuel the nucleus, according to its negative gravity torques. The spiral starts with a large pitch angle from the center and then winds up in a pseudo-ring at the inner Lindblad resonance (ILR) of the nuclear bar. This is the first time that a trailing spiral structure is clearly seen driving the gas inwards inside the ILR ring of the nuclear bar. This phenomenon shows that the massive central black hole has a significant dynamical influence on the gas, triggering its fueling. The gaseous spiral is well correlated with the dusty spiral seen through extinction in HST images, and also with a spiral feature emitting 0.87 mm continuum. This continuum emission must come essentially from cold dust heated by the interstellar radiation field. The HCN(4–3) and HCO⁺(4–3) lines were simultaneously mapped and detected in the nuclear spiral. The HCO⁺(4–3) line is 3 times stronger than the HCN(4–3), as expected when star formation excitation dominates over active galactic nucleus (AGN) heating. The CO(3–2)/HCO⁺(4–3) integrated intensity ratio is ~100. The molecular gas is in remarkably regular rotation, with only slight non-circular motions at the periphery of the nuclear spiral arms. These perturbations are quite small, and no outflow nor AGN feedback is detected.