A spectroscopic study of the near-IR [SIII] lines in a sample of HII galaxies: chemical abundances

Aims.A detailed spectroscopic study covering the blue to near-infrared wavelength range (λ3700 Å-1μm) was performed for a sample of 34 HII galaxies in order to derive fundamental parameters for their HII regions and ionizing sources, as well as gaseous metal abundances. All the spectra included the nebular [SIII]λλ9069,9532 Å lines, given their importance in the derivation of the S/H abundance and relevant ionization diagnostics.
Methods: .A systematic method was followed to correct the near-IR [SIII] line fluxes for the effects of the atmospheric transmission. A comparative analysis of the predictions of the empirical abundance indicators R₂₃ and S₂₃ was performed for our sample galaxies. The relative hardness of their ionizing sources was studied using the η' parameter and exploring the role played by metallicity.
Results: .For 22 galaxies of the sample, a value of the electron temperature T_e[SIII] was derived, along with their ionic and total S/H abundances. Their ionic and total O/H abundances were derived using direct determinations of T_e[OIII]. For the rest of the objects, the total S/H abundance was derived using the S₂₃ calibration. The abundance range covered by our sample goes from 1/20 solar up to solar metallicity. Six galaxies present 12+log (O/H) < 7.8 dex. The mean S/O ratio derived in this work is log (S/O) = -1.68 ± 0.20 dex, 1σ below the solar (S/O)⊙ value. The S/O abundance ratio shows no significant trend with O/H over the range of abundance covered in this work, in agreement with previous findings. There is a trend for HII galaxies with lower gaseous metallicity to present harder ionizing spectra. We compared the distribution of the ionic ratios O⁺/O⁺⁺ vs. S⁺/S⁺⁺ derived for our sample with the predictions of a grid of photoionization models performed for three different stellar effective temperatures. This analysis indicates that a large fraction of galaxies in our sample seem to be ionized by extremely hard spectra, in line with recent suggestions for extra ionizing sources in HII galaxies.