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

Abstract

Aims, A detailed spectroscopic study covering the blue to near-infrared wavelength range (λ3700 A-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 Te[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 Te[OIII]. For the rest of the objects, the total S/H abundance was derived using the S 23 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. © ESO 2006.