The metallicities of luminous, massive field galaxies at intermediate redshifts

We derive oxygen abundances for a sample of 40 luminous (MB≲−19), star-forming, mostly disc, field galaxies with redshifts in the range 0.2 ≲z≲ 0.8, with a median of 〈z〉= 0.45. Oxygen abundances, relative to hydrogen, of the interstellar emitting gas are estimated by means of the empirically calibrated strong emission-line ratio technique. The derived 12 + log (O/H) values range from 8.4 to 9.0, with a median of 8.7. Twenty of these galaxies have securely measured rotation velocities, in the range 50–244 km s−1. The measured emission-line equivalent widths and diagnostic ratios for the intermediate redshift galaxies cover similar ranges to those observed across a large sample of local galaxies. The estimated oxygen abundances for our luminous star-forming intermediate redshift galaxies cover the same range as their local counterparts. However, at a given galaxy luminosity, many of our galaxies have significantly lower oxygen abundances, i.e. 12 + log (O/H) ∼8.6, than local galaxies with similar luminosities. Interestingly, these luminous, massive, intermediate redshift, star-forming galaxies with low oxygen abundances exhibit physical conditions, i.e. emission-line equivalent width and ionization state, very similar to those of local faint and metal-poor star-forming galaxies. The oxygen abundance of the interstellar gas does not seem to correlate with the maximum rotation velocity or the emission scalelength of the parent galaxy. This suggests that there is diversity in the intrinsic properties of the massive field galaxy population at intermediate redshifts. The distribution of the colour excess, derived from the ratio of extinction-uncorrected Hβ and [O ii]λ3727 star formation rate indicators, covers a similar range to that observed locally, but exhibits a lower mean than is observed for local optically selected star-forming galaxies. Luminous field galaxies at intermediate redshifts show similar star formation rates to their local counterparts. However, metal-poor, massive, star-forming galaxies tend to be systematically less affected by internal reddening than metal-rich, massive galaxies, which cover similar range of colour excess to local metal-rich luminous galaxies. Finally, the correlation between oxygen abundance and colour excess for intermediate redshift galaxies is found to be similar to what is observed locally. This result indicates that the dust content of galaxies is more regulated by their chemical evolution rather than by galaxy luminosity.