High-redshift metallicity calibrations for JWST spectra: insights from line emission in cosmological simulations

Abstract

Optical emission-line ratios are traditionally used to estimate gas metallicities from observed galaxy spectra. While such estimators have been calibrated primarily at low redshift, they are commonly used to study high-redshift galaxies, where their applicability may be questioned. We use comprehensive emission-line catalogues of galaxies from the IllustrisTNG simulation including ionization by stars, active nuclei and shocks to reassess the calibrations of both optical and ultraviolet metallicity estimators at redshifts $0\ge z\ge 8$. For present-day galaxies, the predicted optical-line calibrations are consistent with previously published ones, while we find different ultraviolet-line ratios, such as HeII$\hat{I}»$1640/CIII]$\hat{I}»$1908, to provide powerful metallicity diagnostics. At fixed metallicity, most emission-line ratios are predicted to strongly increase or decrease with redshift (with the notable exception of N2O2=[NII]$\hat{I}»$6584/[OII]$\hat{I}»$3727), primarily because of a change in ionization parameter. The predicted dependence of R3=[OIII]$\hat{I}»$5007/H${\hat{I}}^2$ and R23=([OII]$\hat{I}»$3727+[OIII]$\hat{I}»\hat{I}»$4959,5007)/H${\hat{I}}^2$, and to a slightly lesser extent R2=[OII]$\hat{I}»$3727/H${\hat{I}}^2$ and O32=[OIII]$\hat{I}»$5007/[OII]$\hat{I}»$3727, on O abundance for galaxies at $z=4-8$ agrees remarkably well with T${}_e$-based measurements in 14 galaxies observed with JWST. This success leads us to provide new calibrations of optical and ultraviolet metallicity estimators specifically designed for galaxies at z $>$ 4, to guide interpretations of future, high-redshift spectroscopic surveys. We further demonstrate that applying classical z = 0 calibrations to high-redshift galaxies can bias O-abundance estimates downward by up to 1 dex, leading to the conclusion of a stronger evolution of the mass-metallicity relation than the actual one.

16 pages, 5 figures, submitted to MNRAS