We analyse the photometric and spectroscopic properties of four galaxies in
the epoch of reionisation (EoR) within the SMACS 0723 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]
{\lambda}5007 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 roughly 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. We
measure the [O III] {\lambda}5007 rest-frame equivalent widths both directly
from the spectroscopy, and indirectly as inferred from the strong F356W-F444W
colours, finding large [O III] {\lambda}5007 EWs of 400-1000 {\AA}. 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 and colour gradients. 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. Finally, although star-forming galaxies and AGN often occupy similar
regions within the [O III] {\lambda}5007/H\beta-[O II]/H{\delta} plane, we find
that AGN exhibit distinct, red colours in the F150W-F200W, F200W-F277W plane.Comment: 14 pages, 8 figure
