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

TL;DR

This study delivers the first direct measurement of the electron temperature in the low-ionization O$^{+}$ zone of I Zw 18 SE, enabled by detecting the [O II] 7320,7330 auroral doublet in DESI DR1 data. Using PyNeb and auroral-to-nebular line ratios, it derives $T_e$ values for the O$^{+}$, O$^{++}$, and S$^{++}$ zones and a total oxygen abundance of $12+ log(O/H)=7.066 \pm 0.046$, with $\log(N/O)=-1.509 \pm 0.097$ and $\log(S/O)=-1.558 \pm 0.041$. The results test and extend $t_2$-$t_3$ relations to extreme temperatures, showing that the direct $T_e$([O II]) measurement lies between predictions from classic and newer models and highlighting the complexity of temperature structure in extremely metal-poor H II regions. These measurements provide critical anchor points for calibrating metal-poor abundance studies in local and high-redshift galaxies, where direct Te determinations are challenging and biases in empirical relations can impact primordial-parameter estimates.

Abstract

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.

Figures (6)

• Figure 1: Color composite JWST image of I Zw 18 made using four near-IR NIRCam bands (F115W, F200W, F356W, and F444W) by ESA/Webb, NASA, CSA Hirschauer2024. North is up and east is left. Yellow circle represents DESI spectroscopic 1.5" aperture.
• Figure 2: Rest-frame DESI spectrum of I Zw 18 showing clear detection of [OII]$\lambda\lambda$7320,7330 doublet. Gray shared area represents errorbars. Dashed vertical lines mark position of emission lines.
• Figure 3: [OII]$\lambda$3729/[OII]$\lambda$3726 ratio as a function of n$_e$. Blue solid line represents PyNeb model for T$_e = 20\,000$ K, typical for extremely low metallicity HII regions. For comparison, red solid line represents PyNeb model for T$_e = 6\,000$ K. Dotted line and gray area show [OII]$\lambda$3729/[OII]$\lambda$3726 ratio derived from DESI spectrum and its uncertainty.
• Figure 4: T$_e$([OII]) as a function of T$_e$([OIII]) for model from Garnett1992 (solid black line) in comparison with T$_e$([OII]) and T$_e$([OIII]) derived for SE knot in I Zw 18 (black circle). Solid blue line represents quadratic model from MendezDelgado2023 for T$_e$([NII]) as a function of T$_e$([OIII]), while red and green lines represent relations from Cataldi2025 and Pagel1992, respectively. For this model we preserved limited T$_e$([OIII]) representing the range of T$_e$([OIII]) in its calibration sample.
• Figure 5: Observed-to-theoretical Balmer line ratios (H$\alpha$/H$\beta$, H$\gamma$/H$\beta$, H$\delta$/H$\beta$) for two measurement methods. Blue circles represent full spectral fitting including stellar absorption component from FastSpecFit catalog. Orange squares represent our fitting with single Gaussian without subtraction of stellar continuum. Error bars show $1\sigma$ uncertainties, and the dashed line marks the consistency with Case B recombination at n$_e = 100$ cm$^{-3}$ and T$_e = 20\,000$ K.