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 Hβ+[OIII] 4960,5008 Åemitters 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 [OIII] fluxes f[OIII]≳1×10−18 ergs s−1 cm−2. The rest-frame optical line ratios of the median stacked spectrum (median MUV=−19.65+0.59−1.05) indicate negligible dust attenuation, low metallicity (12+log(O/H)=7.2−7.7) and a high ionisation parameter log10U≃−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 overdensity at 7.0<z<7.2 in GOODS-North. The bright end of the UV luminosity function of spectroscopically-confirmed [OIII] emitters is in good agreement with HST dropout-selected samples. Discrepancies between the observed [OIII] LF, [OIII]/UV ratio and [OIII] equivalent widths, and that predicted by theoretical models, suggest burstier star-formation histories and/or more heterogeneous metallicity and ionising conditions in z>7 galaxies. We report a rapid decline of the [OIII] 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% of the ionising budget at z=7−8 (assuming an escape fraction of 10%), raising the prospect of directly detecting a significant fraction of the sources of reionisation with JWST.