Element abundances in metal poor local star-forming galaxies constrained by the weak spectral lines

We have collected from different surveys some significant spectroscopic data observed from star-forming galaxies in the local Universe. The objects showing a relatively rich spectrum in number of lines from different elements were selected in order to constrain the models. In particular, we looked at the relatively weak lines such as [OIII]4363, HeII4686, HeI4471 and HeI5876. We have modelled in detail the spectra by the coupled effect of photoionization from the stars and shocks. We have found that the abundances relative to H of most of the elements are lower than solar but not as low as those evaluated by the direct strong line and the Te methods. Sulfur which appears through the [SII]6717, 6731 and [SIII]6312 lines is not depleted, revealing a strong contribution from the ISM. We have added to the sample the optical-UV spectra of local low-metallicity dwarf galaxies containing the CIV/Hb and CIII]/Hb line ratios in order to determine with relative precision the C/H relative abundance. The results show He/H lower than solar in some objects and suggest that the geometrical thickness of the clouds constrains the HeII/Hb line ratios. We explain the low He/H by mixing of the wind from the star-forming region with ISM clouds.

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