<i>JWST</i> FRESCO: a comprehensive census of H <i>β</i> + [O <scp>iii</scp>] emitters at 6.8 &lt; z &lt; 9.0 in the GOODS fields
R. A. Meyer, Pascal A. Oesch, E. Giovinazzo, Andrea Weibel, Gabriel Brammer, Jorryt Matthee, Rohan P. Naidu, R. J. Bouwens, John Chisholm, Alba Covelo-Paz, Yoshinobu Fudamoto, Michael V. Maseda, Emily R. Nelson, Irene Shivaei, Mengyuan Xiao, Thomas Herard-Demanche, G. D. Illingworth, Josephine Kerutt, Ivan Kramarenko, Ivo Labbé, E. Leonova, D. Magee, Jasleen Matharu, G. Prieto Lyon, Naveen A. Reddy, D. Schaerer, Alice E. Shapley, Mauro Stefanon, M. A. Wozniak, Stijn Wuyts
Abstract
ABSTRACT 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.