2024MNRAS.535.1067M

We present the census of H $\beta$ + [${\rm O\, {\small III}}$] $4960,5008\rm{\mathring{\rm\,A}}$ emitters at $6.8\lt z\lt 9.0$ from the JWST FRESCO survey over 124 arcmin$^2$ in the GOODS-North and GOODS-South fields. Our unbiased spectroscopic search results in 137 spectroscopically confirmed galaxies at $6.8\lt z\lt 9.0$ with observed [${\rm O\, {\small III}}$] fluxes $f_{\rm { [{\rm O\, {\small III}}]}}\gtrsim 1\times 10^{-18}\ \rm {ergs}\ \rm {s}^{-1} \ \rm {cm}^{-2}$. The rest-frame optical line ratios of the median stacked spectrum (median $M_{\rm {UV}}=-19.65^{+0.59}_{-1.05}$) indicate negligible dust attenuation, low metallicity ($12+\log (\rm {O/H})= 7.2-7.7$) and a high ionization parameter $\log _{10}U \simeq -2.5$. We find a factor $\times 1.3$ difference in the number density of $6.8\lt z\lt 9.0$ galaxies between GOODS-South and GOODS-North, which is caused by a single overdensity at $7.0\lt z\lt 7.2$ in GOODS-North. The bright end of the UV luminosity function of spectroscopically confirmed [${\rm O\, {\small III}}$] emitters is in good agreement with HST dropout-selected samples. Discrepancies between the observed [${\rm O\, {\small III}}$] LF, [${\rm O\, {\small III}}$]/UV ratio, and [${\rm O\, {\small III}}$] equivalent widths, and that predicted by theoretical models, suggest burstier star-formation histories and/or more heterogeneous metallicity and ionizing conditions in $z\gt 7$ galaxies. We report a rapid decline of the [${\rm O\, {\small III}}$] luminosity density at $z\gtrsim 6\!-\!7$ which cannot be explained by the evolution of the cosmic star-formation rate density. Finally we find that FRESCO detects in only 2h galaxies likely accounting for $\sim 10-20{{\ \rm per\ cent}}$ of the ionizing budget at $z=7\!-\!8$ (assuming an escape fraction of $10{{\ \rm per\ cent}}$), raising the prospect of directly detecting a significant fraction of the sources of reionization with JWST.