Letter to the Editor

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

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg Mönchhofstraße 12-14 D-69120 Heidelberg, Germany
² Main Astronomical Observatory, National Academy of Sciences of Ukraine 27 Akademika Zabolotnoho St. 03143 Kyiv, Ukraine
³ Instituto de Astrofísica de Andalucía (CSIC) Apartado 3004 18080 Granada, Spain
⁴ Observatório Nacional/MCTIC, R. Gen. José Cristino 77 20921-400 Rio de Janeiro, Brazil
⁵ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto – CAUP Rua das Estrelas PT4150-762 Porto, Portugal
⁶ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ap. 70-264 04510 CDMX, Mexico

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 10 November 2025
Accepted: 12 January 2026

Abstract

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]λλ7320,7330 doublet. We also detected the [O III]λ4363 and [S III]λ6312 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 t₂ − t₃ 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.