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

Free access article

Issue		A&A Volume 457, Number 2, October II 2006


Page(s)		477 - 484
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054488

A&A 457, 477-484 (2006)
DOI: 10.1051/0004-6361:20054488

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

C. Kehrig^{1, 2}, J. M. Vílchez¹, E. Telles², F. Cuisinier³ and E. Pérez-Montero⁴

¹ Instituto de Astrofísica de Andalucía (CSIC), Apartado 3004, 18080 Granada, Spain
    e-mail: kehrig@iaa.es,jvm@iaa.es
² Observatório Nacional, Rua José Cristino 77, 20.921-400 Rio de Janeiro - RJ, Brazil
    e-mail: kehrig@on.br,etelles@on.br
³ GEMAC, Observatório do Valongo/UFRJ, Ladeira do Pedro Antônio 43, 20.080-090 Rio de Janeiro - RJ, Brazil
    e-mail: francois@ov.ufrj.br
⁴ Departamento de Física Teórica, C-XI, Universidad Autónoma de Madrid, 28049 Madrid, Spain
    e-mail: enrique.perez@uam.es

(Received 7 November 2005 / Accepted 6 June 2006 )

Abstract
Aims.A detailed spectroscopic study covering the blue to near-infrared wavelength range ( $\lambda$ 3700 Å-1 $\mu$ m) 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] $\lambda$ $\lambda$ 9069,9532 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 $\eta'$ parameter and exploring the role played by metallicity.
Results.For 22 galaxies of the sample, a value of the electron temperature $T_{\rm e}$ [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 $T_{\rm e}$ [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 $\pm$ 0.20 dex, 1 $\sigma$ below the solar (S/O) $_\odot$ 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.

Key words: ISM: abundances -- HII regions -- galaxies: abundances -- galaxies: dwarf -- galaxies: evolution

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