# The GREATS H$\beta$+[OIII] Luminosity Function and Galaxy Properties at $\mathbf{z\sim8}$: Walking the Way of JWST

## TL;DR

This study derives the [OIII]+Hβ luminosity function at z~8 using combined Spitzer and HST data, providing insights into galaxy properties and predicting JWST spectroscopic yields, highlighting the need for deep pre-imaging.

## Contribution

First to establish the [OIII]+Hβ luminosity function at z~8 using combined photometric data and photoionization models, informing future JWST observations.

## Key findings

- [OIII]+Hβ luminosity function is higher at z~8 than at lower redshifts.
- Strong correlation between [OIII]+Hβ and UV luminosity at z~8.
- Current legacy fields are too shallow for optimal JWST spectroscopy at z~8.

## 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] and H$\beta$ nebular emission lines. To efficiently prepare such observations, we photometrically select a large sample of galaxies at $z\sim8$ 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 [OIII]+H$\beta$ fluxes and equivalent widths for our sample, as well as the average physical properties of $z\sim8$ galaxies, such as the ionizing photon production efficiency with $\log(\xi_\mathrm{ion}/\mathrm{erg}^{-1}\hspace{1mm}\mathrm{Hz})\geq25.77$. We find a relatively tight correlation between the [OIII]+H$\beta$ and UV luminosity, which we use to derive for the first time the [OIII]+H$\beta$ luminosity function (LF) at $z\sim8$. The $z\sim8$ [OIII]+H$\beta$ LF is higher at all luminosities compared to lower redshift, as opposed to the UV LF, due to an increase of the [OIII]+H$\beta$ luminosity at a given UV luminosity from $z\sim3$ to $z\sim8$. Finally, using the [OIII]+H$\beta$ LF, we make predictions for JWST/NIRSpec number counts of $z\sim8$ galaxies. We find that the current wide-area extragalactic legacy fields are too shallow to use JWST at maximal efficiency for $z\sim8$ spectroscopy even at 1hr depth and JWST pre-imaging to $\gtrsim30$ mag will be required.