An investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies

Abstract

Context. We present 8.2 m VLT spectroscopic observations of 28 H ii regions in 16 emission-line galaxies and 3.6m 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. Aims. 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 (L-Z) relation for nearby galaxies, especially at its low-metallicity end and compare it with that for higher-redshift galaxies. Methods. Physical conditions and element abundances in the new sample are derived with the Te-method, excluding six H ii regions from the VLT observations and nearly two third of the H ii regions from the 3.6m observations. Element abundances for the latter galaxies were derived with the semiempirical strong-line method. Results. 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+logO/H=7.05 to 8.22. Our sample includes 27 new galaxies with 12+logO/H< 7.6 which qualify as extremely metal-poor star-forming galaxies (XBCDs). Among them are 10 H ii regions with 12+logO/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 Te-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 g (-12 ≳ Mg ≳ -20) and nearly 2 dex in its oxygen abundance (7.0 lsim; 12+logO/H ≲ 8.8), allowing us to probe the L-Z relation in the nearby universe down to the lowest currently studied metallicity level. The L-Z relation established on the basis of the present sample is consistent with previous ones obtained for emission-line galaxies. . © 2009 ESO.