ALMA imaging of the cold molecular and dusty disk in the type 2 active nucleus of the Circinus galaxy

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
e-mail: konrad.tristram@eso.org
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
⁴ Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
⁵ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁶ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Dept. of Astronomy, Faculty of Science, King Abdulaziz University, PO Box 80203 Jeddah 21589, Saudi Arabia
⁸ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, PR China
⁹ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL University, Sorbonne University, 75014 Paris, France
¹⁰ Observatorio de Madrid, OAN-IGN, Alfonso XII, 3, 28014 Madrid, Spain
¹¹ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
¹² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

Received: 14 March 2022
Accepted: 24 May 2022

Abstract

Context. The central engines of many active galactic nuclei (AGNs) are thought to be surrounded by warm molecular and dusty material in an axisymmetric geometry, thus explaining part of the observational diversity of active nuclei.

Aims. We aim to shed light on the physical properties and kinematics of the molecular material in the nucleus of one of the closest type 2 active galaxies.

Methods. To this end, we obtained high angular resolution Atacama Large Millimeter/submillimeter Array (ALMA) observations of the nucleus of the Circinus galaxy. The observations map the emission at 350 GHz and 690 GHz with spatial resolutions of ∼3.8 pc and ∼2.2 pc, respectively.

Results. The continuum emission traces cold (T ≲ 100 K) dust in a circumnuclear disk with spiral arms on scales of 25 pc, plus a marginally resolved nuclear emission peak. The latter is not extended in polar direction as claimed based on earlier ALMA observations. A significant amount (of the order of 40%) of the 350 GHz emission is not related to dust, but most likely free-free emission instead. We detect CO(3−2) and CO(6−5) as well as HCO⁺(4−3), HCN(4−3), and CS(4−3). The CO emission is extended, showing a spiral pattern, similar to the extended dust emission. Towards the nucleus, CO is excited to higher transitions and its emission is self-absorbed, leading to an apparent hole in the CO(3−2) but not the CO(6−5) emission. On the other hand, the high gas density tracers HCO⁺, HCN, and CS show a strong, yet unresolved (≲4 pc) concentration of the emission at the nucleus, pointing at a very small ‘torus’. The kinematics are dominated by rotation and point at a geometrically thin disk down to the resolution limit of our observations. In contrast to several other AGNs, no HCN enhancement is found towards the nucleus.

Conclusions. The Circinus nucleus is therefore composed of at least two distinct components: (1) an optically thin, warm outflow of ionised gas containing clouds of dust which are responsible for the polar mid-infrared emission, but not seen at submillimetre wavelengths; and (2) a cold molecular and dusty disk, traced by submillimetre continuum and line emission. The latter is responsible for the bulk of the obscuration of the nucleus. These findings support the most recent radiative transfer calculations of the obscuring structures in AGNs, which find a similar two-component structure.