Abstract
<i>Context. <i/>We present 8.2 m VLT spectroscopic observations of 28 H ii regions in 16 emission-line galaxies and 3.6 m ESO telescope spectroscopic observations of 38 H ii regions in 28 emission-line galaxies. These emission-line galaxies were selected mainly from the data release 6 (DR6) of the Sloan digital sky survey (SDSS) as metal-deficient galaxy candidates. <i>Aims. <i/>We collect photometric and high-quality spectroscopic data for a large uniform sample of star forming galaxies including new observations. Our aim is to study the luminosity-metallicity () relation for nearby galaxies, especially at its low-metallicity end and compare it with that for higher-redshift galaxies. <i>Methods. <i/>Physical conditions and element abundances in the new sample are derived with the <i>T<i/><sub>e<sub/>-method, excluding six H ii regions from the VLT observations and nearly two third of the H ii regions from the 3.6 m observations. Element abundances for the latter galaxies were derived with the semiempirical strong-line method. <i>Results. <i/> From our new observations we find that the oxygen abundance in 61 out of the 66 H ii regions of our sample ranges from 12 + log O/H = 7.05 to 8.22. Our sample includes 27 new galaxies with 12 + log O/H < 7.6 which qualify as extremely metal-poor star-forming galaxies (XBCDs). Among them are 10 H ii regions with 12 + log O/H < 7.3. The new sample is combined with a further 93 low-metallicity galaxies with accurate oxygen abundance determinations from our previous studies, yielding in total a high-quality spectroscopic data set of 154 H ii regions. 9000 more galaxies with oxygen abundances, based mainly on the <i>T<i/><sub>e<sub/>-method, are compiled from the SDSS. Photometric data for all galaxies of our combined sample are taken from the SDSS database while distances are from the NED. Our data set spans a range of 8 mag with respect to its absolute magnitude in SDSS <i>g<i/> (–12 <i>M<i/><sub><i>g<i/><sub/> -20) and nearly 2 dex in its oxygen abundance (7.0 12 + log O/H 8.8), allowing us to probe the relation in the nearby universe down to the lowest currently studied metallicity level. The relation established on the basis of the present sample is consistent with previous ones obtained for emission-line galaxies.
Highlights
It was shown more than 20 years ago that low-luminosity dwarf galaxies have systematically lower metallicities compared to more luminous galaxies (Lequeux et al 1979; Skillman et al 1989; Richer & McCall 1995)
From our new observations we find that the oxygen abundance in 61 out of the 66 H ii regions of our sample ranges from 12 + log O/H = 7.05 to 8.22
We present VLT spectroscopic observations of a new sample of 28 H ii regions from 16 emission-line galaxies and ESO 3.6 m telescope spectroscopic observations of a new sample of 38 H ii regions from 28 emission-line galaxies
Summary
It was shown more than 20 years ago that low-luminosity dwarf galaxies have systematically lower metallicities compared to more luminous galaxies (Lequeux et al 1979; Skillman et al 1989; Richer & McCall 1995). This dependence, initially obtained for irregular galaxies, was later confirmed for galaxies of different morphological types The differences between giant and dwarf galaxies are usually attributed to different chemical evolution of galaxies with different masses More efficient mechanisms seem to be at work in massive galaxies converting gas into stars and/or less efficient ones ejecting
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