Model spectra of diffuse X-ray emission in I Zw 18. The plasma model is by APEC, with Z = 0.02 Z_⊙. The thick solid red line represents the model with kT = 0.1 keV and X-ray flux F_X = 2 × 10⁻¹⁵ erg s⁻¹ cm⁻² in the 0.5–10.0 keV band (marked as Chandra in the plot), i.e., the same as reported by Thuan et al. (2004) for the extended X-ray emission. The ISM absorption model component (tbabs) corresponding to the foreground absorption toward I Zw 18, N_H = 1.3 × 10²¹ cm⁻², is applied. The thin dashed red line has the same meaning as the solid red line but for kT = 0.2 keV. The thick solid black line has the same meaning as the solid red line but without accounting for the ISM absorbing column toward I Zw 18. The X-ray luminosity in the 0.3–10 keV band is L_X = 10³⁹ erg s⁻¹. The thin dashed black line has the same meaning as the solid black line but for kT = 0.2 keV.