Luminous, pc-scale CO 6−5 emission in the obscured nucleus of NGC 1377^⋆

¹ Department of Space, Earth and Environment, Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
e-mail: saalto@chalmers.se
² Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, 10617 Taipei, Taiwan
³ Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling, 475 North Charter Street, Madison, WI 53706, USA
⁴ Department of Astrophysics, Astronomy & Mechanics, Faculty of Physics, University of Athens, Panepistimiopolis Zografos 15784, Greece
⁵ Kagoshima University, 890-0065 Kagoshima, Japan
⁶ Observatoire de Paris, LERMA (CNRS: UMR 8112), 61 Av. de l’Observatoire, 75014 Paris, France
⁷ Observatorio Astronómico Nacional (OAN) – Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
⁸ Centre for Star and Planet Formation, Niels Bohr Institute and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark
⁹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹⁰ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹¹ Institut de Radio Astronomie Millimétrique (IRAM), 300 rue de la Piscine, Domaine Universitaire de Grenoble, 38406 St. Martin d’ Hères, France
¹² Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
¹³ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
¹⁴ NRAO, 520 Edgemont Road, Charlottesville, VA 22903, USA
¹⁵ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Kaarina, Finland

Received: 17 February 2017
Accepted: 20 July 2017

Abstract

High-resolution submillimeter line and continuum observations are important in probing the morphology, column density, and dynamics of the molecular gas and dust around obscured active galactic nuclei (AGNs). With high-resolution (0.̋06 × 0.̋05 (6 × 5 pc)) ALMA 690 GHz observations we have found bright (T_B > 80 K) and compact (full width half maximum size (FWHM) size of 10 × 7 pc) CO 6−5 line emission in the nuclear region of the extremely radio-quiet galaxy NGC 1377. The CO 6−5 intensity is partially aligned with the previously discovered jet/outflow of NGC 1377 and is tracing dense (n > 10⁴cm^-3) hot molecular gas at the base of the outflow. The velocity structure is complex and shifts across the jet/outflow are discussed in terms of separate overlapping kinematical components or rotation. High-velocity gas (Δv ± 145km s^-1) is detected inside r < 2−3 pc and we suggest that it is emerging from an inclined rotating disk or torus of position angle PA = 140° ± 20° with a dynamical mass of 3 × 10⁶M_⊙. This mass is consistent with that of a supermassive black hole (SMBH), as inferred from the M−σ relation. The gas mass of the proposed disk/torus constitutes <3% of the dynamical mass inside a radius of 3 pc. In contrast to the intense CO 6−5 line emission, we do not detect 690 GHz dust continuum in the nuclear region of NGC 1377. The upper limit of S(690 GHz) ≲ 2 mJy implies an H₂ column density N(H₂) < 3 × 10²³cm^-2 (averaged in the central 6 × 5 pc beam). This is inconsistent with a Compton thick (CT) source and we discuss the possibility that CT obscuration may instead be occurring on smaller subparsec scales or in a larger foreground structure. From SED fitting we suggest that half of the IR emission of NGC 1377 is nuclear and the rest, mostly the far-infrared (FIR), is emerging from larger scales. The extreme radio quietness, and the lack of emission from other star formation tracers, raise questions on the origin of the FIR emission. We discuss the possibility that it arises from AGN-heated dust along the minor axis.