Validity of strong-line metallicity diagnostics in the high-density, extremely metal-poor regime

Determine whether empirical and theoretical strong-line metallicity diagnostics remain valid for extremely metal-poor galaxies in the high-electron-density regime.

Background

The paper notes that many high-redshift extremely metal-poor galaxy candidates are identified using strong-line diagnostics calibrated primarily with local and relatively metal-rich galaxies. Because high-redshift galaxies may contain substantially denser ionized gas, collisional de-excitation can alter the emission-line ratios on which these diagnostics rely.

Establishing the validity of these diagnostics in the combined extremely metal-poor and high-density regime is important for determining whether candidate galaxies are genuinely metal poor or are high-density impostors whose metallicities have been underestimated.

References

Whether these diagnostics remain valid in the extremely metal-poor and high-density regime is a key unanswered question.

— An Optical Illusion: High Electron Densities Create Extremely Metal-Poor Galaxy Impostors  (2608.20339 - Hsiao et al., 20 Aug 2026) in Section 1, Introduction

However, this simple uniform-shell structure has not yet been verified observationally, and the reverse trend (lower-ionization zones having higher $n_e$) has also been seen \citep[e.g.,][]{Maseda2017,Mingozzi2022}.

— An Optical Illusion: High Electron Densities Create Extremely Metal-Poor Galaxy Impostors  (2608.20339 - Hsiao et al., 20 Aug 2026) in Section 4.4, Density Stratification