  Photoionization models so far are unable to account for the high electron
temperature Te([O III]) implied by the line ratio [O III]4363A/[O III]5007A in
low-metallicity blue compact dwarf galaxies, casting doubts on the assumption
of photoionization by hot stars as the dominant source of heating of the gas in
these objects. Combinations of runs of the 1-D photoionization code NEBU are
used to explore alternative models for the giant H II region shell I Zw 18 NW.
Acceptable models are obtained, which represent schematically an incomplete
shell comprising radiation-bounded condensations embedded in a low-density
matter-bounded diffuse medium. The thermal pressure contrast between gas
components is about a factor 7. The diffuse phase can be in pressure balance
with the hot superbubble fed by mechanical energy from the inner massive star
cluster. The failure of previous modellings is ascribed to (1) the adoption of
an inadequate small-scale gas density distribution, which proves critical when
the collisional excitation of hydrogen contributes significantly to the cooling
of the gas, and possibly (2) a too restrictive implementation of Wolf-Rayet
stars in synthetic stellar cluster spectral energy distributions. A neutral gas
component heated by soft X-rays, whose power is less than 1% of the star
cluster luminosity and consistent with CHANDRA data, can explain the
low-ionization fine-structure lines detected by SPITZER. [O/Fe] is slightly
smaller in I Zw 18 NW than in Galactic Halo stars of similar metallicity and
[C/O] is correlatively large. Extra heating by, e.g., dissipation of mechanical
energy is not required to explain Te([O III]) in I Zw 18. Important
astrophysical developments are at stakes in the 5% uncertainty attached to [O
III] collision strengths.
