Updated indicators of oxygen metallicity for high-$z$ galaxies

TL;DR

The paper tackles the strong scatter and biases that plague classical strong-line oxygen abundance indicators when applied to high-redshift galaxies. By incorporating ionization and nitrogen-enrichment proxies through $O_{32}$ and $N2O2$, and calibrating $12+log(O/H)$ as low-order polynomials of composite indicators, the authors substantially tighten the metallicity–indicator relationships using JWST-derived direct-method abundances. They demonstrate notable improvements in $R^2$ and Spearman correlations for the full high-z sample and find even stronger constraints at $z>2$, with evidence for redshift evolution and a pivot near $z\sim ext{2}$. These results provide a practical and physically motivated framework for precise metallicity measurements in the JWST era, enabling more robust studies of early chemical enrichment and feedback processes.

Abstract

Recent work has demonstrated that widely used strong-line oxygen abundance indicators, such as O3N2, $\rm R23$, and $\widehat{\rm R}$, suffer from large uncertainties when applied to high-redshift galaxies. We show that this loss of precision primarily arises because, at fixed \Oabund, galaxies span a wide dynamic range in ionization parameter and nitrogen enrichment. Here we develop updated indicators that explicitly incorporate both effects via the proxies O32 and N2O2. We define ${\rm R}_{\rm u}\equiv \rm R23+α_1 O32+α_2 N2O2$, $\widehat{\rm R}_{\rm u}\equiv \rm \widehat{R}+β_1 O32+β_2 N2O2$, and ${\rm O}_{\rm u}\equiv \rm O3N2+γ_1 O32+γ_2 N2O2$, and calibrate \Oabund~as low-order polynomials in each composite indicator. Applied to a JWST sample with $T_{\rm e}$-method abundances, the updated indicators substantially tighten the correlations with \Oabund, boosting adjusted coefficients of determination from $\mathbb{R}^2\lesssim 0$ (classical indicators) to $\mathbb{R}^2\gtrsim 0.5$ for the full sample and to $\sim 0.7$ at $z>2$. The residuals reveal a redshift evolution in the mapping between \Oabund, strong lines, ionization, and nitrogen enrichment, with a pivotal turning point near the cosmic noon ($z\sim 2$). Our calibrations provide a practical, physically grounded path to precise metallicity measurements in the JWST era and a firmer basis for quantifying early chemical enrichment and feedback.

Figures (4)

• Figure 1: a) BPT classification of the DJA sample into star-forming (HII), AGN, and composite systems. b) Redshift distribution of the final high-$z$ star-forming sample (median $\bar{z}\sim 2.24$).
• Figure 2: Relations between 12+$\log$(O/H) and classical indicators R23 (top), $\rm \widehat{\rm R}$ (middle), and O3N2 (bottom) for DJA galaxies with direct-method abundances (points). Colored curves denote representative high-$z$ calibrations from the literature. Each panel reports the adjusted coefficient of determination, $\mathbb{R}^2$. Left and right columns are color-coded by O32 and N2O2, respectively.
• Figure 3: Relations between 12+$\log$(O/H) and the updated indicators $\rm R_{\rm u}$ (top), $\rm \widehat{\rm R}_{\rm u}$ (middle), and $\rm O_{\rm u}$ (bottom). Left and right columns show best-fitting linear ($N=1$) and quadratic ($N=2$) models, respectively. Each panel reports adjusted $\mathbb{R}^2$ and Spearman's cc. Color denotes redshift.
• Figure 4: Same as Fig. \ref{['new']}, but restricted to galaxies with $\bar{z}>2.24$. The relations tighten markedly, with adjusted $\mathbb{R}^2$ reaching $\sim 0.7$ for quadratic models.