High-resolution mapping of molecular tori with ALMA

¹ Observatoire de Paris, LUX, Collège de France, CNRS, PSL University, Sorbonne University 75014 Paris, France
² Instituto de Astrofísica de Canarias, Vía Láctea S/N E-38205 La Laguna, Spain
³ Departamento de Astrofísica, Universidad de La Laguna E-38206 La Laguna, Spain
⁴ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII 3 28014 Madrid, Spain
⁵ INAF – Osservatorio Astrofisico di Arcetri Largo Enrico Fermi 5 I-50125 Firenze, Italy
⁶ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory 439 92 Onsala, Sweden
⁷ INAF – Istituto di Radioastronomia Via P. Gobetti 101 40129 Bologna, Italy
⁸ Section of Astrophysics, Astronomy, and Mechanics, Department of Physics, National and Kapodistrian University of Athens, Panepistimioupolis Zografou 15784 Athens, Greece
⁹ Physikalisches Institut, Universität zu Köln Zülpicher Str. 77 50937 Cologne, Germany
¹⁰ Max-Plank-Institut für Radioastronomie, Max-Planck-Gesellschaft Auf dem Hügel 69 53121 Bonn, Germany
¹¹ Institut de Radioastronomie Millimétrique (IRAM) 300 Rue de la Piscine 38400 Saint-Martin-d’Hères, France
¹² European Southern Observatory, Alonso de Córdova 3107 Vitacura Santiago 763-0355, Chile
¹³ Joint ALMA Observatory, Alonso de Córdova 3107 Vitacura Santiago 763-0355, Chile
¹⁴ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4 Roosevelt Rd Taipei 106319, Taiwan
¹⁵ Leiden Observatory, Leiden Univ. PO Box 9513 2300 RA Leiden, The Netherlands
¹⁶ Transdisciplinary Research Area (TRA) ‘Matter’/Argelander-Institut für Astronomie, University of Bonn Bonn, Germany

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Received: 10 September 2025
Accepted: 10 November 2025

Abstract

Recent high-resolution mapping of the circumnuclear regions of active galactic nuclei (AGNs) has revealed the existence of geometrically thin nuclear disks, in general randomly oriented with respect to their galaxy hosts. These molecular tori have typical radii of 10 pc, and contain a few 10⁷ M_⊙ of H₂, with H₂ column densities between 10²³ and 10²⁵ cm⁻². We mapped two of the most massive of these molecular tori with a higher resolution, in order to unveil their morphology and kinematics, their possible warp and clumpiness, and derive their stability and lifetime. We used the highest resolution possible with ALMA (16 km baseline) in Band 7, taking into account for mapping CO(3–2) and HCO⁺(4–3) the compromise between sensitivity and resolution. New features are discovered at the high resolution, obtained with a beam of 0.015″, equivalent to ∼1 pc scale, at their ∼15 Mpc distance. The molecular torus in NGC 613 appears similarly to a ring, depleted in molecular gas near the center. The depletion region is displaced by 3 pc toward the NW from the AGN position, meaning some m = 1 asymmetry in the torus. The molecular torus in NGC 1672 has a different position angle from previous lower-resolution observations, and is edge-on, revealing a geometrically very thin torus (axis ratio 6.5–10), with a clear warp. This confirms that the classical model of a simple geometrically thick dusty torus is challenged by high-resolution observations. The nuclear disks appear to be clumpy, and slightly lopsided. The molecular outflow in NGC 613 is now resolved out. Well inside the sphere of influence of the black holes (BH), we are now able to determine more accurately their mass, for those Seyfert spiral galaxies, in a region of the M-sigma relation where the scatter is maximum.