JWST FRESCO: a comprehensive census of H β + [O iii] emitters at 6.8 < z < 9.0 in the GOODS fields

We present the census of H β + [O III] 4960, 5008 Å emitters at 6.8 < z < 9.0 from the JWST FRESCO survey over 124 arcmin² in the GOODS-North and GOODS-South fields. Our unbiased spectroscopic search results in 137 spectroscopically confirmed galaxies at 6.8 < z < 9.0 with observed [O III] fluxes f _{[O III]} >~ 1 ×10⁻¹⁸ ergs s⁻¹ cm⁻². The rest-frame optical line ratios of the median stacked spectrum (median M_UV = −19. 65^+0.59_−1.05) indicate negligible dust attenuation, low metallicity (12 + log (O/H) = 7.2−7.7) and a high ionization parameter log¹⁰ U ~- −2.5. We find a factor × 1.3 difference in the number density of 6.8 < z < 9.0 galaxies between GOODS-South and GOODS-North, which is caused by a single o v erdensity at 7.0 < z < 7.2 in GOODS- North. The bright end of the UV luminosity function of spectroscopically confirmed [O III] emitters is in good agreement with HST dropout-selected samples. Discrepancies between the observed [O III] LF, [O III]/UV ratio, and [O III] equivalent widths, and that predicted by theoretical models, suggest burstier star-formation histories and/or more heterogeneous metallicity and ionizing conditions in z > 7 galaxies. We report a rapid decline of the [O III] luminosity density at z >~ 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 ∼10−20 per cent of the ionizing budget at z = 7–8 (assuming an escape fraction of 10 per cent ), raising the prospect of directly detecting a significant fraction of the sources of reionization with JWST.