VLT spectroscopy of low-metallicity emission-line galaxies: abundance patterns and abundance discrepancies^⋆,⋆⋆

¹ Max-Planck-Institute for Radioastronomy, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: guseva@mao.kiev.ua
² Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
³ LUTH, Observatoire de Meudon, 92195 Meudon Cedex, France
⁴ Institute for Astrophysics, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁵ Centro de Astrofísica da Universidade do Porto, Rua das Esteral, 4150-762 porto, Portugal
⁶ Department of Astronomy, Oskar Klein Centre, Stockholm University, 10691 Stockholm, Sweden

Received: 9 December 2010
Accepted: 7 March 2011

Abstract

Context. We present deep spectroscopy of a large sample of low-metallicity emission-line galaxies.

Aims. The main goal of this study is to derive element abundances in these low-metallicity galaxies.

Methods. We analyze 121 VLT spectra of H ii regions in 46 low-metallicity emission-line galaxies. Of these spectra 83 are archival VLT/FORS1+UVES spectra of H ii regions in 31 low-metallicity emission-line galaxies that are studied for the first time with standard direct methods to determine the electron temperatures, the electron number densities, and the chemical abundances.

Results. The oxygen abundance of the sample lies in the range 12 + log O/H = 7.2−8.4. We confirm previous findings that Ne/O increases with increasing oxygen abundance, likely because of a higher depletion of oxygen in higher-metallicity galaxies. The Fe/O ratio decreases from roughly solar at the lowest metallicities to about one tenth of solar, indicating that the degree of depletion of iron into dust grains depends on metallicity. The N/O ratio in extremely low-metallicity galaxies with 12 + log O/H < 7.5 shows a slight increase with decreasing oxygen abundance, which could be the signature of enhanced production of primary nitrogen by rapidly rotating stars at low metallicity. We present the first empirical relation between the electron temperature derived from [S iii]λ6312/λ9069 or [N ii]λ5755/λ6583 and the one derived from [O iii]λ4363/λ(4959+5007) in low-metallicity galaxies. We also present an empirical relation between t_e derived from [O ii]λ3727/(λ7320 + λ7330) or [S ii]λ4068/(λ6717  +  λ6730) and [O iii]λ4363/λ(4959+5007). The electron number densities N_e(Cl iii) and N_e(Ar iv) were derived in a number of objects and are found to be higher than N_e(O ii) and N_e(S ii). This has potential implications for the derivation of the pregalactic helium abundance. In a number of objects, the abundances of C⁺⁺ and O⁺⁺ could be derived from recombination lines. Our study confirms the discrepancy between abundances found from recombination lines (RLs) and collisionally excited lines (CELs) and that C/O increases with O/H.