First light and reionisation epoch simulations (FLARES) XI: [O iii] emitting galaxies at 5 < z < 10

JWST has now made it possible to probe the rest-frame optical line emission of high-redshift galaxies extending to z ≈ 9, and potentially beyond. To aid in the interpretation of these emerging constraints, in this work we explore predictions for [O iii]λλ4960, 5008Å emission in high-redshift galaxies using the First Light and Reionisation Epoch Simulations (Flares). We produce predictions for the [O iii] luminosity function, its correlation with the UV luminosity, and the distribution of equivalent widths (EWs). We also explore how the [O iii] EW correlates with physical properties including specific star formation rate, metallicity, and dust attenuation. Our predictions are largely consistent with recent observational constraints on the luminosity function, average equivalent widths, and line ratios. However, they fail to reproduce the observed tail of high-EW sources and the number density of extreme line emitters. Possibilities to explain these discrepancies include an additional source of ionising photons and/or greater stochasticity in star formation in the model or photometric scatter and/or bias in the observations. With JWST now rapidly building larger samples and a wider range of emission lines the answer to this remaining discrepancy should be available imminently.