2008A&A...478..371P

Photoionization models so far are unable to account for the high electron temperature T_e([OIII]) implied by the line intensity ratio [OIII]λ4363Å/[OIII]λ5007Å in low-metallicity blue compact dwarf galaxies, casting doubt on the assumption of photoionization by hot stars as the dominant source of heating of the gas in these objects of large cosmological significance. Combinations of runs of the 1D photoionization code NEBU are used to explore alternative models for the prototype giant HII region shell IZw18NW, with no reference to the filling factor concept and with due consideration for geometrical and stellar evolution constraints. Acceptable models for IZw18NW 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 models 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 IZw18NW 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 T_e([OIII]) in IZw18. Important astrophysical developments depend on the 5% uncertainty attached to [OIII] collision strengths.