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

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.