Shape of the direct-method mass-metallicity relation with JWST
- 1Instituto de Astrofísica de Andalucía (IAA-CSIC)
Abstract
Aims. We investigate the mass-metallicity relation (MZR) from z = 1 to z = 9 using electron-temperature based gas-phase metallici-ties. We explore how auroral-line selection effects, SFHs, and secondary abundances impact its interpretation in the early Universe. Methods. We compiled a homogeneous sample of 286 star-forming galaxies observed with JWST/NIRSpec medium-resolution spectroscopy, selected through detections of the [O III] λ4363 auroral line from the public DAWN JWST Archive (DJA). We derived the electron densities, temperatures, and oxygen abundances using the direct Te method, along with relative N/O and He/H abundances. Stellar masses are obtained via SED fitting and star formation rates from reddening-corrected Balmer emission lines. To quantify any auroral-line selection biases, we additionally stacked sets of galaxy spectra with and without auroral-line detections, extending the MZR into regimes that are inaccessible with respect to individual measurements. Results. The auroral line-detected sample spans stellar masses of log(M★/M⊙) = 6.77-10.5, star formation rates of log(SFR[M⊙ yr−1]) = −0.24 to 2.38, and low metallicities of 12 + log(O/H) = 6.9-8.4, with electron temperatures of T[OIII] = 10 030 K to 37 400 K and electron densities ranging from 100 to 990 cm−3. A linear fit to the individually detected sample yields a MZR slope of γ = 0.38 ± 0.09. Stacked galaxies without individual auroral-line detections define a relation with a similar slope, but a systematically higher metallicity by 0.2–0.3 dex at a fixed stellar mass. These two sequences differ markedly in their star-formation properties: galaxies with individual auroral-line detections exhibit higher SFRs, larger emission-line equivalent widths, and larger deviations from the fundamental metallicity relation (FMR). In contrast, stacked non-detections appear more chemically evolved and closer to the FMR. In several stacked bins, we also found enhanced N/O and He/H ratios, suggesting rapid enrichment channels such as Wolf–Rayet stars or supermassive stars. Conclusions. The MZR of high-z galaxies traced by JWST at low stellar masses is shaped both by recent SFH and auroral-line selection effects. Auroral lines preferentially identify high-EW and high-sSFR galaxies in the low-metallicity envelope of the relation, where the effects of dilution due to inflows of pristine gas are greater; whereas non-detections reveal a more enriched sequence closer to the metallicity predicted by the FMR.
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BibTeX
@article{gimenezalcazar2026shape,
title = {{Shape of the direct-method mass-metallicity relation with JWST}},
author = {Giménez-Alcázar A. and Amorín R. and Vilchez J. M.},
journal = {Astronomy \& Astrophysics},
year = {2026},
month = nov,
doi = {10.1051/0004-6361/202660477},
url = {https://doi.org/10.1051/0004-6361/202660477},
note = {CC BY},
}
RIS
TY - JOUR
TI - Shape of the direct-method mass-metallicity relation with JWST
AU - Giménez-Alcázar A.
AU - Amorín R.
AU - Vilchez J. M.
PY - 2026
DA - 2026/11/04
JO - Astronomy & Astrophysics
DO - 10.1051/0004-6361/202660477
UR - https://doi.org/10.1051/0004-6361/202660477
AB - Aims. We investigate the mass-metallicity relation (MZR) from z = 1 to z = 9 using electron-temperature based gas-phase metallici-ties. We explore how auroral-line selection effects, SFHs, and secondary abundances impact its interpretation in the early Universe. Methods. We compiled a homogeneous sample of 286 star-forming galaxies observed with JWST/NIRSpec medium-resolution spectroscopy, selected through detections of the [O III] λ4363 auroral line from the public DAWN JWST Archive (DJA). We derived the electron densities, temperatures, and oxygen abundances using the direct Te method, along with relative N/O and He/H abundances. Stellar masses are obtained via SED fitting and star formation rates from reddening-corrected Balmer emission lines. To quantify any auroral-line selection biases, we additionally stacked sets of galaxy spectra with and without auroral-line detections, extending the MZR into regimes that are inaccessible with respect to individual measurements. Results. The auroral line-detected sample spans stellar masses of log(M★/M⊙) = 6.77-10.5, star formation rates of log(SFR[M⊙ yr−1]) = −0.24 to 2.38, and low metallicities of 12 + log(O/H) = 6.9-8.4, with electron temperatures of T[OIII] = 10 030 K to 37 400 K and electron densities ranging from 100 to 990 cm−3. A linear fit to the individually detected sample yields a MZR slope of γ = 0.38 ± 0.09. Stacked galaxies without individual auroral-line detections define a relation with a similar slope, but a systematically higher metallicity by 0.2–0.3 dex at a fixed stellar mass. These two sequences differ markedly in their star-formation properties: galaxies with individual auroral-line detections exhibit higher SFRs, larger emission-line equivalent widths, and larger deviations from the fundamental metallicity relation (FMR). In contrast, stacked non-detections appear more chemically evolved and closer to the FMR. In several stacked bins, we also found enhanced N/O and He/H ratios, suggesting rapid enrichment channels such as Wolf–Rayet stars or supermassive stars. Conclusions. The MZR of high-z galaxies traced by JWST at low stellar masses is shaped both by recent SFH and auroral-line selection effects. Auroral lines preferentially identify high-EW and high-sSFR galaxies in the low-metallicity envelope of the relation, where the effects of dilution due to inflows of pristine gas are greater; whereas non-detections reveal a more enriched sequence closer to the metallicity predicted by the FMR.
SN - 0004-6361
SN - 1432-0746
ER -