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

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 λ8449 emission feature. Based on the detection of OI λ8449 in a stack of objects with non-detections, we infer that this feature is ubiquitous among star-forming galaxies at about 0.5% the strength of H α. 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 λ8449 relative to H α in these objects. We evaluate four production mechanisms: recombination, collisional excitation, Ly β fluorescence, and stellar continuum fluorescence. Of these mechanisms, stellar continuum fluorescence (rather than Ly β fluorescence) provides the most compelling explanation for the OI λ8449 emission based on the additional detection of the near-infrared OI λ11290 and OI λ13168 emission lines. Adopting physical conditions derived from a stacked composite spectrum, we make predictions for the contribution of the other mechanisms to OI λ8449, determining that recombination, collisional excitation, and Ly β fluorescence contribute negligibly to this feature. While a simple HII region model using Cloudy reproduces the average observed OI λ8449 /H α 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.