JWST FRESCO: a comprehensive census of Hβ+[OIII] emitters at 6.8<z<9.0 in the GOODS fields

This is the authors' abstract. We don't add key points for this paper.

We present the census of Hβ+[O III] $4960,5008$ 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\ erg\ s⁻¹ \ cm⁻². The rest-frame optical line ratios of the median stacked spectrum indicate negligible dust attenuation, low metallicity (12+log(O/H)= 7.2-7.7) and a high ionisation parameter log₁₀U -2.5 at a median UV magnitude MUV=-19.65+0.59-1.05. 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 single overdensity 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 that from pre-JWST dropout-selected samples. Discrepancies between the observed [O III] LF, [O III] /UV ratio and [O III] equivalent widths distribution 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 [O III] luminosity density at z 6-7 which cannot be explained solely by the evolution of the cosmic star-formation rate density. Finally, we find that FRESCO, in only $2$h, captures star-forming galaxies likely accounting for ~ 10-20% of the ionising budget at $z=7$ and $z=8$, raising the prospect of detecting directly all the sources of reionisation with JWST.