On the geometrical evolution of the ionized gas in HII galaxies

In this paper, we investigate the behaviour of the number of Lyman continuum ionizing photons as compared to the actual number of hydrogen recombinations in HII galaxies. We evaluate the number of ionizing photons from the population synthesis of spectra observed in the visible, extrapolating the spectra to the extreme ultraviolet (EUV), beyond the Lyman limit. We check for possible systematic deviations of the predicted ionizing spectra in the EUV by comparing the ratio of the predicted number of ionizing photons to the number of recombinations, as measured in H$\beta$, ${\rm \Delta \log Q(H^0)}$, with the metallicity. We find that, as far as the number of ionizing photons is concerned, no systematic tendency can be detected. The ${\rm H\beta}$ equivalent width can be understood as a nebular age indicator, decreasing with age, although the observed ${\rm H\beta}$ equivalent width can also be affected by the contribution to the continuum by the accumulation of previous, non-ionizing stellar populations. We attribute the increase of ${\rm \Delta \log Q(H^0)}$ with the age of the burst to the fact that more and more ionizing photons escape the nebulae when the nebulae get older, because of their increasing, expansion-induced subfragmentation.