First direct electron temperature measurement in [O II] zone in I Zw 18

We present new precise measurements of electron temperatures and oxygen abundances in the southeast knot of I Zw 18, one of the most metal-poor blue compact dwarf galaxies known, using spectroscopic data from the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1). For the first time in I Zw 18, we directly measure electron temperature in the low-ionization zone using the rarely detected [O II] $λ\lambda7320,7330$ doublet. We also detect the auroral lines [O III] $\lambda4363$ and [S III] $\lambda6312$, associated with high and intermediate ionization zones, respectively. We derive $T_{\mathrm{e}}([\mathrm{O},\mathrm{III}])=21,200\pm860\ \mathrm{K}$, $T_{\mathrm{e}}([\mathrm{O},\mathrm{II}])=16,170\pm950\ \mathrm{K}$, and $T_{\mathrm{e}}([\mathrm{S},\mathrm{III}])=17,290\pm1750\ \mathrm{K}$, highlighting a significant temperature difference between ionization zones. Using these direct temperature measurements, we determine a total oxygen abundance of $12+\log(\mathrm{O}/\mathrm{H})=7.066\pm0.046$, $\log(\mathrm{N}/\mathrm{O})=-1.509\pm0.097$, and $\log(\mathrm{S}/\mathrm{O})=-1.558\pm0.041$. Our results extend the calibration of $t_2$–$t_3$ relations to the highest temperatures, providing important anchor points for the temperature structure of extremely metal-poor H II regions, including high-redshift galaxies where direct temperature measurements are especially challenging.

💡 Research Summary

This paper presents the first direct measurement of the electron temperature in the low‑ionization zone of the extremely metal‑poor blue compact dwarf galaxy I Zw 18, using data from the Dark Energy Spectroscopic Instrument (DESI) Data Release 1. The authors focus on the southeast (SE) star‑forming knot, observed through a 1.5″ aperture that corresponds to ~138 pc at the galaxy’s distance. By employing the Fast SpecFit pipeline, they obtain high‑quality emission‑line fluxes with signal‑to‑noise ratios > 5 for all relevant lines, including the rarely detected auroral