What heats the bright HII regions in I Zw 18?

Abstract

We have used the radiation field from a starburst population synthesis model appropriate for the brightest HII region of IZw 18 to perform a photoionization model analysis of this object. We have investigated whether, with the observed nebular density distribution as revealed by the HST images and a total stellar radiation compatible with the observed UV flux, the constraints represented by the observed ionization structure, the HeII/Hbeta ratio, the Halpha flux and the electron temperature indicated by the [OIII] 4363/5007 ratio can be reproduced. We have found that, even taking into account strong deviations from the adopted spectral energy distribution of the ionizing radiation and the effect of additional X-rays, the photoionization models yield too low a 4363/5007 ratio by about 30 %. This discrepancy is significant and poses an interesting problem, which cannot be solved by expected inaccuracies in the atomic data. The missing energy may be of the same order of magnitude as the one provided by the stellar photons or lower, depending on the way it acts on the 4363 line. Elemental abundance determinations in I Zw 18 are affected by this problem. Before the nature of the missing heating source and its interactions with the nebular gas are better understood it is, however, not possible to estimate the typical uncertainties by which standard empirical methods are affected. Other topics discussed include: nebular HeII emission, photon leakage, [OI] 6300 emission, reddening, ionization correction factors. (abridged abstract)