Spectrophotometric properties of galaxies at intermediate redshifts (z ~ 0.2–1.0)

II. The Luminosity – Metallicity relation

¹ Laboratoire d'Astrophysique de Toulouse et Tarbes (LATT - UMR 5572), Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France e-mail: flamare@ast.obs-mip.fr
² Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Strasse 1 Postfach 1317, 85741 Garching, Germany
³ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁴ Institut d'Astrophysique de Paris, CNRS, 98 bis boulevard Arago, 75014 Paris, France

Received: 9 June 2005
Accepted: 29 October 2005

Abstract

We present the gas-phase oxygen abundance (O/H) for a sample of 131 star-forming galaxies at intermediate redshifts (). The sample selection, the spectroscopic observations (mainly with VLT/FORS) and associated data reduction, the photometric properties, the emission-line measurements, and the spectral classification are fully described in a companion paper (Paper I). We use two methods to estimate the O/H abundance ratio: the “standard” method which is based on empirical calibrations, and the CL01 method which is based on grids of photo-ionization models and on the fitting of emission lines. For most galaxies, we have been able to solve the problem of the metallicity degeneracy between the high- and low-metallicity branches of the O/H vs. relationship using various secondary indicators. The luminosity – metallicity () relation has been derived in the B- and R-bands, with metallicities derived with the two methods ( and CL01). In the analysis, we first consider our sample alone and then a larger one which includes other samples of intermediate-redshift galaxies drawn from the literature. The derived relations at intermediate redshifts are very similar (same slope) to the relation obtained for the local universe. Our sample alone only shows a small, not significant, evolution of the relation with redshift up to . We only find statistical variations consistent with the uncertainty in the derived parameters. Including other samples of intermediate-redshift galaxies, we find however that galaxies at appear to be metal-deficient by a factor of compared with galaxies in the local universe. For a given luminosity, they contain on average about one third of the metals locked in local galaxies.