Letter to the Editor

Structures Of Dust and gAs (SODA): Constraining the innermost dust properties of II Zw96 with JWST observations of H₂O and CO

¹ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
e-mail: igbernete@gmail.com
² Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
³ Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
⁴ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus

Received: 27 November 2023
Accepted: 27 December 2023

Abstract

We analyze JWST NIRSpec+MIRI/MRS observations of the infrared (IR) gas-phase molecular bands of the most enshrouded source (D1) within the interacting system and luminous IR galaxy II Zw 096. We report the detection of rovibrational lines of H₂O ν₂ = 1 − 0 (∼5.3–7.2 μm) and ¹²CO ν = 1 − 0 (∼4.45–4.95 μm) in D1. The CO band shows the R- and P-branches in emission and the spectrum of the H₂O band shows the P-branch in emission and the R-branch in absorption. The H₂O R-branch in absorption unveils an IR-bright embedded compact source in D1 and the CO broad component features a highly turbulent environment. From both bands, we also identified extended intense star-forming (SF) activity associated with circumnuclear photodissociation regions (PDRs), consistent with the strong emission of the ionised 7.7 μm polycyclic aromatic hydrocarbon band in this source. By including the 4.5–7.0 μm continuum information derived from the H₂O and CO analysis, we modelled the IR emission of D1 with a dusty torus and SF component. The torus is very compact (diameter of ∼3 pc at 5 μm) and characterised by warm dust (∼370 K), giving an IR surface brightness of ∼3.6 × 10⁸ L_⊙ pc⁻². This result suggests the presence of a dust-obscured active galactic nucleus (AGN) in D1, which has an exceptionally high covering factor that prevents the direct detection of AGN emission. Our results open a new way to investigate the physical conditions of inner dusty tori via modelling the observed IR molecular bands.