Surprising increase of electron temperature in metal-rich star-forming region

TL;DR

This study reveals an unexpected increase in electron temperature in metal-rich star-forming regions, challenging existing theories and the reliability of the direct $T_e$ method for metallicity determination.

Contribution

It uncovers a surprising trend of rising electron temperature in metal-rich galaxies, contradicting theoretical expectations and prompting a re-evaluation of the direct $T_e$ method.

Findings

Electron temperature increases at high metallicity in certain ions.

The trend is consistent across different datasets and analysis methods.

No clear physical explanation for the high [OII] line ratios was found.

Abstract

The electron temperature is a crucial parameter for the determination of the gas-phase metallicity of galaxies. Low electron temperature is expected for metal-rich galaxies, theoretically. We report the discovery that temperature, as measured through auroral-to-strong line ratios of O$^+$, trends in reverse directions at 12+log(O/H) $\geq$ 8.7. This trend remains consistent regardless of the emission line fitting method employed and is not attributable to contamination or dust attenuation correction. Notably, this phenomenon is not observed in other low-ionization ions, such as S$^+$ and N$^+$, which also probe electron temperature. The results are verified in two independent datasets. We analyze the potential cause for the high [OII] auroral-to-strong line ratios at high metallicities, finding that no specific reason could account for that. This finding challenges the fundamental principles of the direct $T_e$ method for metallicity measurement, warranting further investigation into its physical interpretation.