An investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies *,**
Max-Planck-Institute for Radioastronomy, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: email@example.com
2 Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
3 Instituto de Astrofísica de Andalucía (CSIC), Camino Bajo de Huétor 50, Granada 18008, Spain
4 Department of Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden
5 Astronomisches Rechen-Institut am Zentrum für Astronomie (ZAH), Mönchhofstr. 12-14, 69120 Heidelberg, Germany
6 Institute for Astrophysics, University of Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
Accepted: 18 July 2009
Context. 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.
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 () 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 3.6 m 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 + 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 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 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.
Key words: galaxies: abundances / galaxies: starburst / galaxies: ISM / galaxies: fundamental parameters
© ESO, 2009