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] $\lambda \lambda 7320,7330$ doublet. We also detect the auroral lines [O III] $\lambda 4363$ and [S III] $\lambda 6312$, associated with high and intermediate ionization zones, respectively. We derive $T_{\mathrm{e}}([\mathrm{O}\mathrm{I}\mathrm{I}\mathrm{I}])=21200\pm 860\mathrm{K}$, $T_{\mathrm{e}}([\mathrm{O}\mathrm{I}\mathrm{I}])=16170\pm 950\mathrm{K}$, and $T_{\mathrm{e}}([\mathrm{S}\mathrm{I}\mathrm{I}\mathrm{I}])=17290\pm 1750\mathrm{K}$, highlighting a significant temperature difference between ionization zones. Using these direct temperature measurements, we determine a total oxygen abundance of $12+\log (\mathrm{O}\mathrm{/}\mathrm{H})=7.066\pm 0.046$, $\log (\mathrm{N}\mathrm{/}\mathrm{O})=-1.509\pm 0.097$, and $\log (\mathrm{S}\mathrm{/}\mathrm{O})=-1.558\pm 0.041$. 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.