Structures Of Dust and gAs (SODA): Constraining the innermost dust properties of II Zw96 with JWST observations of H${}_2$O 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 H${}_2$O $\hat{I}{½}_2$=1-0 ($\sim$5.3-7.2 $\hat{I}¼$m) and ${}^{12}$CO $\hat{I}½$=1-0 ($\sim$4.45-4.95 $\hat{I}¼$m) in D1. The CO band shows the R- and P-branches in emission and the spectrum of the H${}_2$O band shows the P-branch in emission and the R-branch in absorption. The H${}_2$O 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 $\hat{I}¼$m polycyclic aromatic hydrocarbon band in this source. By including the 4.5-7.0 $\hat{I}¼$m continuum information derived from the H${}_2$O and CO analysis, we modelled the IR emission of D1 with a dusty torus and SF component. The torus is very compact (diameter of $\sim$3 pc at 5 $\hat{I}¼$m) and characterised by warm dust ($\sim$ 370 K), giving an IR surface brightness of $\sim$3.6$\times$10${}^8$ L${}_{\mathrm{sun}}$/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.

Accepted for publication in A&A (official date of acceptance: 27/12/2023; submitted 27/11/2023). 11 pages, 10 figures