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

Abstract

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 [OII]-selected, M_B,AB<-21.5, z~1.4 galaxies, by which we measured Hbeta and [OIII]5007 emission line fluxes from J-band spectra, and Halpha line fluxes plus upper limits for [NII]6584 fluxes from H-band spectra. The z~1.4 galaxies are characterized by the high [OIII]/[OII] line ratios, low extinction and low metallicity 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 [OIII]/[OII]) is seen in progressively more luminous galaxies as the redshift increases. These spectra are caused by a combination of high ionisation parameter q and lower [O/H]. Pegase2 chemical evolution models are used to relate the observed metallicities and luminosities of z~1.4 galaxies to galaxy samples at lower and higher redshift. Not surpringsingly, 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 progressively in 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 [OIII]/[OII] or low [O/H]) are seen in progressively more luminous (massive) systems at higher redshifts.