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

TL;DR

This study analyzes weak spectral lines in local star-forming galaxies to determine element abundances, revealing lower-than-solar levels for most elements and insights into the ISM's role and cloud geometry effects.

Contribution

It provides detailed modeling of weak spectral lines to constrain element abundances and the physical conditions in local star-forming galaxies, incorporating shock effects and ISM contributions.

Findings

Most element abundances are lower than solar but higher than direct methods suggest.

Sulfur abundance indicates significant ISM contribution.

He/H ratios are sometimes below solar, influenced by cloud geometry and stellar winds.

Abstract

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.