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

We present new precise measurements of the electron temperatures and oxygen abundances in the southeast knot of I Zw 18, one of the most metal poor blue compact dwarf galaxies known. We used spectroscopic data from the Dark Energy Spectroscopic Instrument Data Release 1 (DESI DR1). For the first time in I Zw 18, we directly measured the electron temperature in the low-ionization zone using the rarely detected [O II ] λλ 7320,7330 doublet. We also detected the [O III ] λ 4363 and [S III ] λ 6312 auroral lines, which are associated with high- and intermediate-ionization zones, respectively. We derived T e ([O III ]) = 21 200 ± 860 K, T e ([O II ]) = 16 170 ± 950 K, and T e ([S III ]) = 17 290±1750, which highlights a significant temperature difference between the ionization zones. Using these direct temperature measurements, we determined a total oxygen abundance of 12+log(O/H) = 7.066 ± 0.046, log(N/O) = –1.509 ± 0.097, and log(S/O) = –1.558 ± 0.041. Our results extend the calibration of t 2 − t 3 relations to the highest temperatures and provide important anchor points for the temperature structure of extremely metal-poor H II regions, including high-redshift galaxies, for which direct temperature measurements are especially challenging.