The matallicities of star-forming galaxies at intermediate redshifts 0.47 < z < 0.92

Abstract

Estimates of the [O/H] metallicity of the star-forming gas in a sample of 66 CFRS galaxies between 0.47 < z < 0.92 have been made based on the flux ratios of bright emission lines. Most galaxies (> 75%) have the [O/H] ~ 8.9 metallicities that are seen locally in galaxies of similar luminosities. However, a minority (< 25%) appear to have significantly lower metallicities [O/H] < 8.6 as indicated by high values of the Pagel R23 parameter. The high metallicities of the majority of the galaxies suggest that they do not fade to be low metallicity dwarf galaxies today. Only one of the 66 galaxies has an optical emission line spectrum similar to the few Lyman break galaxies recently observed at z ~ 3, emphasizing the differences with that population. The inferred emission line gas [O/H] metallicity broadly correlates with luminosity in both the rest B- and J-bands but with considerable scatter introduced. The metallicity does not appear to correlate well with galaxy size, H-beta strength or, with the very limited data available, the kinematics. The metallicity does correlate well with the continuum optical-infrared colors in a way that could be explained as a combination of effects, none of which should produce large variations in the M/L ratio. These results support a "down-sizing" picture of galaxy evolution rather than a "fading dwarf" picture in which the luminous active galaxies at high redshift are highly brightened dwarf galaxies. The overall change in metallicity of star-forming galaxies over the last half of the age of the Universe appears to have been modest, Delta[O/H] = 0.08 +/- 0.06. This is consistent with the age-metallicity relation in the Galactic disk and is broadly consistent with models for the chemical evolution of the Universe, especially those that consider different environments.