Near-infrared spectroscopy of 0.4 < z < 1.0 CFRS galaxies:: Oxygen abundances, SFRs, and dust

Using new J-band VLT ISAAC and Keck NIRSPEC spectroscopy, we have measured H alpha and [N (II)] lambda 6584 line fluxes for 0.47 < z < 0.92 CFRS galaxies that have [O (II)] lambda 3727, H beta, and [O (III)] lambda 5007 line fluxes available from optical spectroscopy to investigate how the properties of the star-forming gas in galaxies evolve with redshift. We derive the extinction and oxygen abundances for the sample using a method based on a set of ionization parameter and oxygen abundance diagnostics, simultaneously fitting the [O (II)], H beta, [O (III)], H beta, and [N (II)] line fluxes. The individual reddening measurements allow us to accurately correct the H alpha-based star formation rate (SFR) estimates for extinction. Our most salient conclusions are as follows: (1) in all 30 CFRS galaxies, the source of the gas ionization is not due to AGN activity; ( 2) we find a range of 0 < AV < 3, suggesting that it is important to determine the extinction for every single galaxy in order to reliably measure SFRs and oxygen abundances in high-redshift galaxies; (3) high values of [N (II)] lambda 6584/H alpha > 0.1 for most (but not all) of the CFRS galaxies indicate that they lie on the high-metallicity branch of the R-23 calibration; (4) about one-third of the 0.47 < z < 0.92 CFRS galaxies in our sample have lower metallicities than local galaxies with similar luminosities and star formation rates; (5) comparison with a chemical evolution model indicates that these low-metallicity galaxies are unlikely to be the progenitors of metal-poor dwarf galaxies at z similar to 0.