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Spectrophotometric properties of galaxies at intermediate redshifts (z ~ 0.2--1.0) II. The Luminosity -- Metallicity relation

We present the gas-phase oxygen abundance (O/H) for a sample of 131 star-forming galaxies at intermediate redshifts ($0.2<z<1.0$). 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" $R\_{\mathrm{23}}$ 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. $R\_{\mathrm{23}}$ relationship using various secondary indicators. The luminosity -- metallicity ($L-Z$) relation has been derived in the $B$- and $R$-bands, with metallicities derived with the two methods ($R\_{\mathrm{23}}$ 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 $L-Z$ relations at intermediate redshifts are very similar (same slope) to the $L-Z$ relation obtained for the local universe. Our sample alone only shows a small evolution of the $L-Z$ relation with redshift up to $z\sim1.0$. 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 $z\sim1$ appear to be metal-deficient by a factor of $\sim3$ 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.