Molecular line emission in NGC 1068 imaged with ALMA^⋆

I. An AGN-driven outflow in the dense molecular gas

¹ Observatorio Astronómico Nacional (OAN)-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 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
⁴ Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 St.Martin d’ Hères, France
⁵ Department of Physics and Astronomy, UCL, Gower Place, London WC1E 6BT, UK
⁶ Instituto de Física de Cantabria, CSIC-UC, 39005 Santander, Spain
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁸ Max-Planck-Institut für Astronomie, Königstuhl, 17, 69117 Heidelberg, Germany
⁹ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
¹⁰ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse, France
¹¹ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
¹² Centro de Astrobiología (CSIC-INTA), Ctra de Torrejón a Ajalvir,, km 4, 28850 Torrejón de Ardoz Madrid, Spain
¹³ Instituto de Astrofísica de Andalucía (CSIC), Apdo 3004, 18080 Granada, Spain
¹⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁶ Astronomy Department, King Abdulazizi University, PO Box 80203, 21589 Jeddah, Saudi Arabia
¹⁷ Institute for Astronomy, Department of Physics, ETH Zurich, 8093 Zurich, Switzerland
¹⁸ Instituto de Astrofísica de Canarias, Calle vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
¹⁹ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
²⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
²¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
²² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 19 March 2014
Accepted: 4 June 2014

Abstract

Aims. We investigate the fueling and the feedback of star formation and nuclear activity in NGC 1068, a nearby (D = 14 Mpc) Seyfert 2 barred galaxy, by analyzing the distribution and kinematics of the molecular gas in the disk. We aim to understand if and how gas accretion can self-regulate.

Methods. We have used the Atacama Large Millimeter Array (ALMA) to map the emission of a set of dense molecular gas (n(H₂) ≃ 10^{5 − 6} cm^-3) tracers (CO(3–2), CO(6–5), HCN(4–3), HCO⁺(4–3), and CS(7–6)) and their underlying continuum emission in the central r ~ 2 kpc of NGC 1068 with spatial resolutions ~0.3″ − 0.5″ (~20–35 pc for the assumed distance of D = 14 Mpc).

Results. The sensitivity and spatial resolution of ALMA give an unprecedented detailed view of the distribution and kinematics of the dense molecular gas (n(H₂) ≥ 10^{5 − 6}cm^-3) in NGC 1068. Molecular line and dust continuum emissions are detected from a r ~ 200 pc off-centered circumnuclear disk (CND), from the 2.6 kpc-diameter bar region, and from the r ~ 1.3 kpc starburst (SB) ring. Most of the emission in HCO⁺, HCN, and CS stems from the CND. Molecular line ratios show dramatic order-of-magnitude changes inside the CND that are correlated with the UV/X-ray illumination by the active galactic nucleus (AGN), betraying ongoing feedback. We used the dust continuum fluxes measured by ALMA together with NIR/MIR data to constrain the properties of the putative torus using CLUMPY models and found a torus radius of 20⁺⁶_-10pc. The Fourier decomposition of the gas velocity field indicates that rotation is perturbed by an inward radial flow in the SB ring and the bar region. However, the gas kinematics from r ~ 50 pc out to r ~ 400 pc reveal a massive (M_mol~ 2.7^+0.9_-1.2 × 10⁷ M_⊙) outflow in all molecular tracers. The tight correlation between the ionized gas outflow, the radio jet, and the occurrence of outward motions in the disk suggests that the outflow is AGN driven.

Conclusions. The molecular outflow is likely launched when the ionization cone of the narrow line region sweeps the nuclear disk. The outflow rate estimated in the CND, dM/dt~ 63⁺²¹_-37 M_⊙ yr^-1, is an order of magnitude higher than the star formation rate at these radii, confirming that the outflow is AGN driven. The power of the AGN is able to account for the estimated momentum and kinetic luminosity of the outflow. The CND mass load rate of the CND outflow implies a very short gas depletion timescale of ≤1 Myr. The CND gas reservoir is likely replenished on longer timescales by efficient gas inflow from the outer disk.