JWST FRESCO: a comprehensive census of H${\hat{I}}^2$+[OIII] emitters at 6.8<z<9.0 in the GOODS fields

Abstract

We present the census of H${\hat{I}}^2$+[OIII] 4960,5008 Ã emitters at 6.8<z<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 $6.8<z<9.0$ with observed [OIII] fluxes $f_{[OIII]}\gtrsim 1\times 10^{-18}\mathrm{ergs}{\mathrm{s}}^{-1}{\mathrm{cm}}^{-2}$. The rest-frame optical line ratios of the median stacked spectrum (median $M_{\mathrm{UV}}=-19.65_{-1.05}^{+0.59}$) indicate negligible dust attenuation, low metallicity ($12+\log (O/H)=7.2-7.7$) and a high ionisation parameter ${\log }_{10}U\simeq -2.5$. We find a factor $\times 1.3$ 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 $z\gtrsim 6-7$ 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 $\sim 10-20\%$ 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.

20 pages + appendices. Accepted in MNRAS. Public catalogue release at https://github.com/rameyer/fresco. V3: matching accepted version