Abstract
(abridged) We present deep spectroscopy of a large sample of low-metallicity emission-line galaxies. The main goal of this study is to derive element abundances in these low-metallicity galaxies. We analyze 121 VLT spectra of HII regions in 46 low-metallicity emission-line galaxies. 83 of these spectra are archival VLT/FORS1+UVES spectra of HII 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. The oxygen abundance of the sample lies in the range 12 + log O/H = 7.2-8.4. The Ne/O ratio increases with increasing oxygen abundance. 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+logO/H<7.5 shows a slight increase with decreasing oxygen abundance. We present the first empirical relation between the electron temperature derived from [SIII]6312/9069 or [NII]5755/6583 and the one derived from [OIII]4363/(4959+5007) in low-metallicity galaxies. 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.
Highlights
Comprehensive studies of low-metallicity emission-line galaxies in the nearby universe are essential to investigate star formation and galaxy evolution with chemical conditions close to those in the early universe
We find that the lower limit of N/O in H ii regions with lowest metallicity is higher than the above-mentioned plateau value of −1.6, implying that there is some increase in N/O with decreasing oxygen abundance at 12 + log O/H < 7.5 (Fig. 12a)
We have presented an analysis of archival Very Large Telescope (VLT)/FORS1+UVES spectroscopic observations of a large sample of low-metallicity emission-line galaxies
Summary
Comprehensive studies of low-metallicity emission-line galaxies in the nearby universe are essential to investigate star formation and galaxy evolution with chemical conditions close to those in the early universe. We obtain a large VLT sample of 121 H ii regions, which includes to our knowledge all high-quality spectra from low-metallicity objects observed with the VLT/UVES+FORS instruments. They are used to study the abundance patterns of low-metallicity emission-line galaxies. The total sample consists of objects with low extinction and in general with a high equivalent width EW(Hβ) of the Hβ emission line, which suggests a young age of the respective starburst It includes almost all most metal-deficient galaxies known, which allows for the deepest insights into the abundance pattern of the lowestmetallicity galaxies available in the local Universe.
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