James Webb Space Telescope Feed Post

Harvard ADS: JWST FRESCO: a comprehensive census of H\beta+[OIII] emitters at 6.8 < z < 9.0 in the GOODS fields

Paper abstract: We present the census of H\beta+[O III] 4960,5008 \AA 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 [O III] fluxes f_{[O III]}\gtrsim 1\times 10^{-18}\ \rm{erg}\ \rm{s}^{-1} \ \rm{cm}^{-2}. The rest-frame optical line ratios of the median stacked spectrum indicate negligible dust attenuation, low metallicity (12+\log(\rm{O/H})= 7.2-7.7) and a high ionisation parameter \log_{10}U ~eq -2.5 at a median UV magnitude M_{\rm{UV}}=-19.65^{+0.59}_{-1.05}. 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 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\gtrsim 6-7 which cannot be explained solely by the evolution of the cosmic star-formation rate density. Finally, we find that FRESCO, in only 2h, 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.