Letter to the Editor

The Galaxy Activity, Torus, and Outflow Survey (GATOS)

III. Revealing the inner icy structure in local active galactic nuclei

¹ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
e-mail: igbernete@gmail.com
² Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
³ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
⁴ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
⁵ Max-Planck-Institut fur extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
⁶ Instituto de Radioastronomía and Astrofísica (IRyA-UNAM), 3-72 (Xangari), 8701 Morelia, Mexico
⁷ Instituto de Astrofísica de Canarias, Calle Vía Láctea, s/n, 38205 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
⁹ Departamento de Física de la Tierra y Astrofísica, Fac. de CC Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁰ Instituto de Física de Partículas y del Cosmos IPARCOS, Fac. de CC Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹¹ Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA 91125, USA
¹² Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 92190 Meudon, France
¹³ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
¹⁴ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
¹⁵ Department of Physics & Astronomy, University of Alaska Anchorage, AK 99508-4664, USA
¹⁶ Department of Physics & Astronomy, University of Southampton, Hampshire, SO17 1BJ Southampton, UK
¹⁷ Telespazio UK for the European Space Agency (ESA), ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
¹⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁹ Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), Stanford University, Stanford, CA 94305, USA
²⁰ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Avenida Ejercito Libertador 441, Santiago, Chile
²¹ The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA
²² Astronomical Observatory, Volgina 7, 11060 Belgrade, Serbia
²³ Centre for Extragalactic Astronomy, Durham University, South Road, Durham DH1 3LE, UK

Received: 13 October 2023
Accepted: 1 December 2023

Abstract

We use JWST/MIRI MRS spectroscopy of a sample of six local obscured type 1.9/2 active galactic nuclei (AGN) to compare their nuclear mid-IR absorption bands with the level of nuclear obscuration traced by X-rays. This study is the first to use subarcsecond angular resolution data of local obscured AGN to investigate the nuclear mid-IR absorption bands with a wide wavelength coverage (4.9–28.1 μm). All the nuclei show the 9.7 μm silicate band in absorption. We compare the strength of the 9.7 and 18 μm silicate features with torus model predictions. The observed silicate features are generally well explained by clumpy and smooth torus models. We report the detection of the 6 μm dirty water ice band (i.e., a mix of water and other molecules such as CO and CO₂) at subarcsecond scales (∼0.26″ at 6 μm; inner ∼50 pc) in a sample of local AGN with different levels of nuclear obscuration in the range log N_H^X-Ray (cm⁻²)∼22 − 25. We find good correlation between the 6 μm water ice optical depths and N_H^X-Ray. This result indicates that the water ice absorption might be a reliable tracer of the nuclear intrinsic obscuration in AGN. The weak water ice absorption in less obscured AGN (log N_H^X-ray (cm⁻²)≲23.0 cm⁻²) might be related to the hotter dust temperature (> T_sub^H₂O ∼ 110 K) expected to be reached in the outer layers of the torus due to their more inhomogeneous medium. Our results suggest it might be necessary to include the molecular content, such as H₂O, aliphatic hydrocarbons (CH−), and more complex polycyclic aromatic hydrocarbon (PAH) molecules, in torus models to better constrain key parameters such as the torus covering factor (i.e., nuclear obscuration).