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What heats the bright H II regions in I Zw 18?
G. Stasiska 1 and
D. Schaerer 2
Received 17 March 1999 / Accepted 8 September 1999
We have used the radiation field from a starburst population synthesis model appropriate for the brightest H II region of I Zw 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, one could reproduce the constraints represented by the observed ionization structure, the He II 4686/H ratio, the H flux and the electron temperature indicated by the [O III ] 4363/5007 ratio.
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 [O III ] 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 [O III ] 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.
Several side-products of our photoionization analysis of I Zw 18 are the following:
Key words: galaxies: abundances galaxies: individual: I Zw 18 galaxies: ISM galaxies: starburst stars: early-type stars: Wolf-Rayet
Send offprint requests to: G. Stasiska
Online publication: November 2, 1999