Seeing sharper and deeper: JWST's first glimpse of the photometric and spectroscopic properties of galaxies in the epoch of reionization

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 similar to 8 galaxies with high equivalent width (EW) [O iii] lambda 5007 emission and z similar to 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 approximate to 0.5 dex) from the local correlation between [O iii] lambda 5007/H beta and [Ne iii] lambda 3869/[O ii], which is consistent with the predictions from simulations of high-redshift galaxies having elevated line-excitation ratios. We measure the [O iii] lambda 5007 rest-frame EWs both directly from the spectroscopy, and indirectly as inferred from the strong F356W-F444W colours, finding large [O iii] lambda 5007 EWs of 225-1740 angstrom. The [O iii] lambda 5007 and H beta 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.