2023MNRAS.523.3423T

We analyse the photometric and spectroscopic properties of four galaxies in the epoch of reionization (EoR) within the SMACS J0723.3-7327 JWST Early Release Observations field. Given the known spectroscopic redshifts of these sources, we investigated the accuracy with which photometric redshifts can be derived using NIRCam photometry alone, finding that F115W imaging is essential to distinguish between z ∼ 8 galaxies with high equivalent width (EW) [O III] λ5007 emission and z ∼ 10 Balmer break galaxies. We find that all four sources exhibit strong (≥0.6 mag) F356W-F444W colours, which sit at the extreme end of theoretical predictions from numerical simulations. We find that these galaxies deviate (by ≈0.5 dex) from the local correlation between [O III] λ5007/H β and [Ne III] λ3869/[O II], which is consistent with the predictions from simulations of high-redshift galaxies having elevated line-excitation ratios. We measure the [O III] λ5007 rest-frame EWs both directly from the spectroscopy, and indirectly as inferred from the strong F356W-F444W colours, finding large [O III] λ5007 EWs of 225-1740 Å. The [O III] λ5007 and H β EWs are consistent with those seen in extreme, intensely star-forming dwarf galaxies in the local Universe. Our structural analysis indicates that these galaxies are resolved, exhibiting irregular shapes with bright clumps. In line with the predictions from the FLARES hydrodynamic simulations, such intense star formation and extreme nebular conditions are likely the norm, rather than the exception, in the EoR.