The AURORA Survey: Determining the Production Mechanism for OI $\mathbf{\lambda 8449}$ Emission in Star-forming Galaxies at Cosmic Noon

Abstract

We analyze deep JWST/NIRSpec observations of 58 star-forming galaxies at $1.3<z<4.8$ in the AURORA survey to investigate the origin of the permitted OI $\lambda 8449$ emission feature. Based on the detection of OI $\lambda 8449$ in a stack of objects with non-detections, we infer that this feature is ubiquitous among star-forming galaxies at $\sim$0.5% the strength of H$\alpha$. We additionally compare our results with measurements of this line in local HII regions from the CHAOS survey and star-forming galaxies at $z<0.1619$ in the DESI survey, finding a similar strength of OI $\lambda 8449$ relative to H$\alpha$ in these objects. We evaluate four production mechanisms: recombination, collisional excitation, Ly$\beta$ fluorescence, and stellar continuum fluorescence. Of these mechanisms, stellar continuum fluorescence (rather than Ly$\beta$ fluorescence) provides the most compelling explanation for the OI $\lambda 8449$ emission based on the additional detection of the near-infrared OI $\lambda 11290$ and OI $\lambda 13168$ emission lines. Adopting physical conditions derived from a stacked composite spectrum, we make predictions for the contribution of the other mechanisms to OI $\lambda 8449$, determining that recombination, collisional excitation, and Ly$\beta$ fluorescence contribute negligibly to this feature. While a simple HII region model using Cloudy reproduces the average observed OI $\lambda 8449$/H$\alpha$ ratios in the AURORA sample, future works focused on refining model density profiles will be a valuable step in better explaining the near-infrared OI emission-line strengths.