Shape of the direct-method mass-metallicity relation with JWST: Fast-Track Nitrogen and Helium Enrichment

@misc{gimenezalcazar2026shape,
  title = {{Shape of the direct-method mass-metallicity relation with JWST: Fast-Track Nitrogen and Helium Enrichment}},
  author = {A. Giménez-Alcázar and R. Amorín and J. M. Vilchez},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2605.05327},
  eprint = {2605.05327},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2605.05327},
}

TY  - UNPB
TI  - Shape of the direct-method mass-metallicity relation with JWST: Fast-Track Nitrogen and Helium Enrichment
AU  - A. Giménez-Alcázar
AU  - R. Amorín
AU  - J. M. Vilchez
PY  - 2026
DA  - 2026/10/07
PB  - arXiv
DO  - 10.48550/arxiv.2605.05327
UR  - https://arxiv.org/abs/2605.05327
AB  - We investigate the mass-metallicity relation (MZR) from z=1 to z=9 using electron-temperature-based gas-phase metallicities and examine how auroral-line selection, star-formation history, and secondary abundances affect its interpretation in the early Universe. We compile 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 derive electron densities, temperatures, and oxygen abundances using the direct T_e 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 auroral-line selection biases, we additionally stack galaxy spectra with and without auroral-line detections, extending the MZR into regimes inaccessible to individual measurements. The auroral-line-detected sample spans log(M*/Msun)=6.77-10.5 and 12+log(O/H)=6.9-8.4. A linear fit gives an MZR slope of γ=0.38 ± 0.09. Stacked galaxies without individual λ4363 detections define a relation with a similar slope but metallicities higher by ~0.2-0.3 dex at fixed stellar mass. Auroral-line detections also show higher SFRs, larger equivalent widths, and larger offsets from the fundamental metallicity relation, whereas non-detections appear more chemically evolved and closer to it. Several stacked bins also show enhanced N/O and He/H ratios. These results indicate that the low-mass high-redshift MZR traced by JWST is shaped by both recent star-formation history and auroral-line selection effects. Auroral lines preferentially identify high-EW, high-sSFR galaxies in the low-metallicity envelope, whereas non-detections reveal a more enriched sequence closer to the metallicity expected from the FMR.
ER  -