Surprising Increase of Electron Temperature in Metal-Rich Star-Forming Regions

Measuring gas-phase metallicity, the relative abundance of elements heavier than helium to the abundance of hydrogen, is essential for constraining the chemical evolution of galaxies. The electron temperature is a crucial parameter for the determination of metallicity as the strongest emission lines from metal ions are all collisionally excited, which depends sensitively on temperature. Electron temperatures can be measured by comparing the strengths of two emission lines of the same ion that originate from two different upper levels, which are usually referred to as auroral-to-strong line ratios. Low electron temperature is theoretically expected for metal-rich star-forming regions, as metal ions in high metallicity gas provide efficient cooling through collisional excitation and radiative de-excitation. In this work, we report the discovery that temperature, as measured through auroral-to-strong line ratios of O ⁺ , trends in reverse directions at supersolar metallicities. 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. This finding challenges the fundamental principles of the direct T _e method for metallicity measurement, warranting further investigation into its physical interpretation.