[Paper Review] Direct T_e-based Metallicities of z=2-9 Galaxies with JWST/NIRSpec: Empirical Metallicity Calibrations Applicable from Reionization to Cosmic Noon

Ryan L. Sanders, Alice E. Shapley+3 more|arXiv (Cornell University)|Mar 14, 2023

Stellar, planetary, and galactic studies20 citations

TL;DR

The paper presents detections of [O iii]4364 auroral lines in a large JWST/NIRSpec sample, derives Te-based metallicities, and constructs the first high-redshift empirical calibrations for strong-line ratios applicable from reionization to cosmic noon.

ABSTRACT

We report detections of the [OIII]$λ$4364 auroral emission line for 16 galaxies at z=2.1-8.7, measured from JWST/NIRSpec observations obtained as part of the Cosmic Evolution Early Release Science (CEERS) survey program. We combine this CEERS sample with 9 objects from the literature at z=4-9 with auroral-line detections from JWST/NIRSpec and 21 galaxies at z=1.4-3.7 with auroral-line detections from ground-based spectroscopy. We derive electron temperature T_e and direct-method oxygen abundances for the combined sample of 46 star-forming galaxies at z=1.4-8.7. We use these measurements to construct the first high-redshift empirical T_e-based metallicity calibrations for the strong-line ratios [OIII]/H$β$, [OII]/H$β$, R23=([OIII]+[OII])/H$β$, [OIII]/[OII], and [NeIII]/[OII]. These new calibrations are valid over 12+log(O/H)=7.0-8.4 and can be applied to samples of star-forming galaxies at z=2-9, leading to an improvement in the accuracy of metallicity determinations at Cosmic Noon and in the Epoch of Reionization. The high-redshift strong-line relations are offset from calibrations based on typical $z\sim0$ galaxies or HII regions, reflecting the known evolution of ionization conditions between $z\sim0$ and $z\sim2$. Deep spectroscopic programs with JWST/NIRSpec promise to improve statistics at the low and high ends of the metallicity range covered by the current sample, as well as improve the detection rate of [NII]$λ$6585 to allow the future assessment of N-based indicators. These new high-redshift calibrations will enable accurate characterizations of metallicity scaling relations at high redshift, improving our understanding of feedback and baryon cycling in the early universe.

Motivation & Objective

• Motivate robustly measuring gas-phase metallicities across cosmic history to understand gas flows and baryon cycling.
• Provide direct-method metallicities for a sizable high-redshift sample using JWST auroral lines.
• Develop empirical calibrations linking rest-optical strong-line ratios to Te-based O/H at high redshift.

Proposed method

• Detect [O iii]4364 auroral lines in JWST/NIRSpec CEERS data to obtain Te and direct O/H.
• Combine with JWST literature and ground-based auroral-line samples to assemble 46 galaxies at z=1.4-8.7.
• Compute electron temperature Te from [O iii]4364/[O iii]5008 (or 1666/5008) and electron density ne from [S ii].
• Calculate O/H directly and fit empirical relations between strong-line ratios (O3, O2, R23, O32, Ne3O2) and Te-based O/H.
• Provide calibrations valid over 12+log(O/H)=7.0–8.4 for use in z=2-9 samples.

Experimental results

Research questions

• RQ1 Can high-redshift galaxies yield reliable Te-based metallicities using JWST/NIRSpec auroral lines?
• RQ2 What are the empirical relations between Te-based O/H and common rest-optical strong-line ratios at high redshift?
• RQ3 How do high-z calibrations compare to local calibrations given evolving ISM conditions?
• RQ4 Can a unified high-z calibration be applied from Cosmic Noon to the Epoch of Reionization (z~9)?

Key findings

• Detected Te-based metallicities for a combined sample of 46 star-forming galaxies spanning z=1.4-8.7.
• Constructed high-redshift empirical calibrations for O3/Hβ, O2/Hβ, R23, O3O2, and Ne3O2 in the metallicity range 12+log(O/H)=7.0-8.4.
• High-redshift strong-line relations are offset from z~0 calibrations, reflecting evolution in ISM ionization conditions.
• Calibrations are applicable to z=2-9 and can improve metallicity determinations during Cosmic Noon and the Epoch of Reionization.
• ISMs at high z show harder ionizing spectra and elevated electron densities, consistent with the need for redshift-specific calibrations.
• Future JWST programs may improve statistics and enable assessment of N-based indicators.