ALMA captures feeding and feedback from the active galactic nucleus in NGC 613^⋆

¹ Observatoire de Paris, LERMA, CNRS, PSL Univ., Sorbonne University, UPMC, Paris, France
e-mail: anelise.audibert@obspm.fr
² Collège de France, 11 Pl. Marcelin Berthelot, 75231 Paris, France
³ Observatorio Astronómico Nacional (OAN-IGN)-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 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁷ INAF – Isitituto di Radioastronomia, via Piero Gobetti 101, 40129 Bologna, Italy
⁸ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁹ IRAM, 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 Dep. of Astrophysics, Astronomy & Mechanics, Faculty of Physics, National and Kapodistrian University of Athens, 15784 Panepistimiopolis Zografou, Greece
¹² National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Penteli, 15236 Athens, Greece
¹³ Leiden Observatory, Leiden Univ., PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁴ European Southern Observatory, Alonso de Córdova, 3107, Vitacura Santiago, 763-0355, Chile
¹⁵ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile

Received: 4 May 2019
Accepted: 1 August 2019

Abstract

We report ALMA observations of CO(3–2) emission in the Seyfert/nuclear starburst galaxy NGC 613, at a spatial resolution of 17 pc, as part of our NUclei of GAlaxies (NUGA) sample. Our aim is to investigate the morphology and dynamics of the gas inside the central kiloparsec, and to probe nuclear fueling and feedback phenomena. The morphology of CO(3–2) line emission reveals a two-arm trailing nuclear spiral at r ≲ 100 pc and a circumnuclear ring at a radius of ∼350 pc that is coincident with the star-forming ring seen in the optical images. Also, we find evidence for a filamentary structure connecting the ring and the nuclear spiral. The ring reveals two breaks into two winding spiral arms corresponding to the dust lanes in the optical images. The molecular gas in the galaxy disk is in a remarkably regular rotation, however the kinematics in the nuclear region are very skewed. The nuclear spectrum of CO and dense gas tracers HCN(4–3), HCO⁺(4–3), and CS(7–6) show broad wings up to ±300 km s⁻¹, associated with a molecular outflow emanating from the nucleus (r ∼ 25 pc). We derive a molecular outflow mass M_out = 2 × 10⁶ M_⊙ and a mass outflow rate of Ṁ_out = 27 M_⊙ yr⁻¹. The molecular outflow energetics exceed the values predicted by AGN feedback models: the kinetic power of the outflow corresponds to P_K, out = 20%L_AGN and the momentum rate is Ṁ_outv ∼ 400L_AGN/c. The outflow is mainly boosted by the AGN through entrainment by the radio jet, but given the weak nuclear activity of NGC 613, we might be witnessing a fossil outflow resulting from a previously strong AGN that has now faded. Furthermore, the nuclear trailing spiral observed in CO emission is inside the inner Lindblad resonance ring of the bar. We compute the gravitational torques exerted in the gas to estimate the efficiency of the angular momentum exchange. The gravity torques are negative from 25 to 100 pc and the gas loses its angular momentum in a rotation period, providing evidence for a highly efficient inflow towards the center. This phenomenon shows that the massive central black hole has significant dynamical influence on the gas, triggering the inflowing of molecular gas to feed the black hole.