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

Abstract

The James Webb Space Telescope will allow to spectroscopically study an unprecedented number of galaxies deep into the reionization era, notably by detecting [OIII]ΛΛ4959, 5007, and Hβ nebular emission lines. To efficiently prepare such observations, we photometrically select a large sample of galaxies at z∼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] + Hβ fluxes and equivalent widths for our sample, as well as the average physical properties of z ∼ 8 galaxies, such as the ionizing photon production efficiency with log(ξion/erg-1 Hz) ≥ 25.77. We find a relatively tight correlation between the [O III] + Hβ and UV luminosity, which we use to derive for the first time the [O III]ΛΛ4959, 5007 + Hβ luminosity function (LF) at z ∼ 8. The z ∼ 8 [O III] + Hβ LF is higher at all luminosities compared to lower redshift, as opposed to the UV LF, due to an increase of the [O III] + Hβ luminosity at a given UV luminosity from z ∼ 3 to z ∼ 8. Finally, using the [O III] + Hβ LF, we make predictions for JWST/NIRSpec number counts of z ∼ 8 galaxies.We find that the current wide-area extragalactic legacy fields are too shallow to use JWST at maximal efficiency for z ∼ 8 spectroscopy even at 1 h depth and JWST pre-imaging to 30 mag will be required.