Oxygen Gas Abundances at z~1.4: Implications for the Chemical Evolution History of Galaxies^*

The 1 < z < 2 redshift window hosts the peak of the star formation and metal production rates. Studies of the metal content of the star-forming galaxies at these epochs are, however, sparse. We report VLT ISAAC near-infrared spectroscopy for a sample of five [O II]-selected, M_B,AB ≲ -21.5, z ~ 1.4 galaxies, by which we measured Hβ and [O III] λ5007 emission-line fluxes from J-band spectra, and Hα line fluxes plus upper limits for [N II] λ6584 fluxes from H-band spectra. The z ~ 1.4 galaxies are characterized by the high [O III]/[O II] line ratios, low extinctions, and low metallicities that are typical of lower luminosity CADIS galaxies at 0.4 ≲ z ≲ 0.7 and of more luminous Lyman break galaxies at z ~ 3 but not seen in CFRS galaxies at 0.4 ≲ z ≲ 0.9. This type of spectrum (e.g., high [O III] λ5007/[O II] λ3727) is seen in progressively more luminous galaxies as the redshift increases. These spectra are caused by a combination of high-ionization parameter q and lower [O/H]. PÉGASE2 chemical evolution models are used to relate the observed metallicities and luminosities of z ~ 1.4 galaxies to galaxy samples at lower and higher redshifts. Not surprisingly, we see a relationship between redshift and inferred chemical age. We suppose that the metal-enriched reservoirs of star-forming gas that we are probing at intermediate redshifts are being mostly consumed to build up both the disk and the bulge components of spiral galaxies. Finally, our analysis of the metallicity-luminosity relation at 0 ≲ z ≲ 1.5 suggests that the period of rapid chemical evolution may take place in progressively lower mass systems as the universe ages. These results are consistent with a "downsizing"-type picture, in the sense that particular signatures (e.g., high [O III]/[O II] or low [O/H]) are seen in progressively more luminous (massive) systems at higher redshifts.