The GREATS Hβ plus [O III] luminosity function and galaxy properties at z ∼ 8: walking the way of JWST

The James Webb Space Telescope will allow to spectroscopically study an unprecedented number of galaxies deep into the reionization era, notably by detecting [O III]lambda lambda 4959, 5007, and H beta nebular emission lines. To efficiently prepare such observations, we photometrically select a large sample of galaxies at z similar to 8 and study their rest-frame optical emission lines. Combining data from the GOODS Re-ionization Era wide-Area Treasury from Spitzer (GREATS) survey and from HST we perform spectral energy distribution (SED) fitting, using synthetic SEDs from a large grid of photoionization models. The deep Spitzer/IRAC data combined with our models exploring a large parameter space enables to constrain the [O III] + H beta fluxes and equivalent widths for our sample, as well as the average physical properties of z similar to 8 galaxies, such as the ionizing photon production efficiency with log(xi(ion)/erg(-1) Hz) >= 25.77. We find a relatively tight correlation between the [O III] + H beta and UV luminosity, which we use to derive for the first time the [O III]lambda lambda 4959, 5007 + H beta luminosity function (LF) at z similar to 8. The z similar to 8 [O III] + H beta LF is higher at all luminosities compared to lower redshift, as opposed to the UV LF, due to an increase of the [O III] + H beta luminosity at a given UV luminosity from z similar to 3 to z similar to 8. Finally, using the [O III] + H beta LF, we make predictions for JWST/NIRSpec number counts of z similar to 8 galaxies. We find that the current wide-area extragalactic legacy fields are too shallow to use JWST at maximal efficiency for z similar to 8 spectroscopy even at 1 h depth and JWST pre-imaging to similar to 30 mag will be required.