James Webb Space Telescope Feed Post

Harvard ADS: A theoretical investigation of far-infrared fine structure lines at z>6 and of the origin of the [OIII]88/[CII]158 enhancement

Paper abstract: [Abridged] The [OIII]_{88\mu m}/[CII]_{158\mu m} and [OIII]_{88\mu m}/[NII]_{122\mu m} luminosity ratios have shown to be promising tracers of the ionisation state and gas-phase metallicity of the ISM. Observations of galaxies at redshift z > 6 show peculiarly higher [OIII]_{88\mu m}/[CII]_{158\mu m} luminosity ratios compared to local sources. No model has so far successfully managed to match the observed emission from both [OIII]_{88\mu m} and [CII]_{158\mu m} as well as their ratio. We use Cloudy to model the [CII]_{158\mu m}, [OIII]_{88\mu m}, [NII]_{122\mu m} and [NIII]_{57\mu m} emission lines of Ponos: a high-resolution (m_{\mathrm{gas}} = 883.4\, M_{\odot}) cosmological zoom-in simulation of a galaxy at redshift z = 6.5, which is post-processed using kramses-rt. We modify Carbon, Nitrogen and Oxygen abundances in our Cloudy models to obtain C/O and N/O abundance ratios respectively lower and higher than Solar, more in line with recent high-z observational constraints. We find [OIII]_{88\mu m}/[CII]_{158\mu m} luminosity ratios that are a factor of ~ 5 higher compared to models assuming solar abundances. Additionally, we find an overall better agreement of the simulation with high-z observational constraints of the [CII]_{158\mu m}-SFR and [OIII]_{88\mu m}-SFR relations. This shows that a lower C/O abundance ratio is essential to reproduce the enhanced [OIII]_{88\mu m}/[CII]_{158\mu m} luminosity ratios observed at z > 6. By assuming a super-solar N/O ratio, motivated by recent z > 6 JWST observations, our models yield an [OIII]_{88\mu m}/[NII]_{122\mu m} ratio of 1.3, which, according to current theoretical models, would be more appropriate for a galaxy with a lower ionisation parameter than the one we estimated for Ponos.