SPATIAL INHOMOGENEITIES IN GIANT EXTRAGALACTIC H-II REGIONS

The spatial distribution of relevant parameters for a sample of giant extragalactic H II regions (GEHR) has been analyzed. The results indicate that within the errors associated either with the measurements or with the use of metallicity calibrators, the metallicity Z (coded as O/H) is constant across the explored area of a given GEHR. The same applies to the He/H and Ne/O abundance ratios. All the GEHR were found to have He/H = 0.075 within 30%, without any trend with Z. On its side the Ne/O ratios for different GEHR show a trend with the integrated equivalent width of H-beta. That trend is attributed not to differences in the Ne/O abundance ratio, but to technical problems associated with the ionization correction factor for Ne. We also analyzed the possible causes for the point-to-point spectral variations within a GEHR. We found that they cannot be due to fluctuations in T(eff) since this parameter shows no changes over 5000 K, what is not enough to explain the observations. Thus, the spatial distribution of the line ratios has to be attributed to fluctuations in the ionization parameter U. We also argue that these variations are not produced by inhomogeneities in the electronic density, but are associated with the aging and evolution of the ionizing stellar clusters. A fact which has not yet been taken into account in photoionization models.