The Metallicities 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 Canada-France Redshift Survey (galaxies in the range 0.47 < z < 0.92 have been made based on the flux ratios of bright emission lines. The spectra of the galaxies are very similar to those of typical galaxies in the local universe. 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 R 23 parameter. The fraction of the sample at these lower [O/H] values would be reduced if the reddening in these objects were lower than the E B-V ∼ 0.3 assumed. 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 from 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, especially at lower luminosities, by the range in [O/H] observed. The metallicity does not appear to correlate well with galaxy size, Hβ strength, or, with the very limited data available, kinematics. The metallicity does correlate well with the continuum optical and optical to infrared colors in a way that could be explained as a combination of photospheric effects, differing ages of the stellar population, and/or different amounts of reddening. None of these should produce large variations in the M/L ratio. These results support a " downsizing " picture of galaxy evolution, in which the manifestations of star-forming evolutionary activity appear in progressively more massive galaxies at earlier epochs, 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, Δ[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.