Mass-metallicity relation of zCOSMOS galaxies at z ≈ 0.7, its dependence on star formation rate, and the existence of massive low-metallicity galaxies⋆

Aims. The knowledge of the number and physical nature of low-metallicity massive galaxies is crucial for determining and interpreting the mass-metallicity relation (MZR). Methods. Using VLT-ISAAC near-infrared (NIR) spectroscopy of 39 zCOSMOS 0.5 < z < 0.9 galaxies, we have measured Hα and [N II] λ 6584 emission line fluxes for galaxies with [O II] λ 3727, Hβ, and [O III] λ 5007 available from VIMOS optical spectroscopy. The NIR spectroscopy enables us to break the degeneracy of the R23 method to unambiguously derive O/H gas metallicities and star formation rates (SFRs) from extinction-corrected Hα fluxes. Results. We used the position in the D4000 vs. [O III] λ 5007/Hβ diagram of galaxies with reliable O/Hs from NIR spectroscopy as a benchmark. This enabled us to break the lower or upper branch R23 degeneracy of additional 900 zCOSMOS galaxies at 0.5 < z < 0.9, which do not have measurements of Hα and [N II] λ 6584. Additionally, the Hα-based SFR measurements were used to find the best SFR calibration based on [O II] λ 3727 for the 0.5 < z < 0.9 zCOSMOS galaxies without Hα measurements. With a larger zCOSMOS sample of star-forming galaxies at z ≈ 0.7 with reliable O/H and SFR measurements, we studied the MZR at z ≈ 0.7 and its dependence on (specific) SFR. We find a fraction of 19% of lower mass 9.5 < log (M/M ) < 10.3 zCOSMOS galaxies that shows a larger evolution of the MZR relation than is found in higher mass galaxies; they are more metal poor at a given mass by a factor of 2−3 than SDSS. This indicates that the low-mass MZR slope steepens at z ≈ 0.7 compared to local galaxies. Many of these galaxies with lower metallicity would be missed by studies that assume upper branch R23 solution for all galaxies. The existence of these metal-poor galaxies at z ≈ 0.7 can be interpreted as the chemical version of galaxy downsizing. Moreover, the sample of zCOSMOS galaxies shows direct evidence that the SFR influences the MZR at these redshifts. The measured metallicities for the zCOSMOS sample broadly agree with the values expected for a non-evolving fundamental metallicity relation. This comparison also reveals that galaxies with lower metallicities and typically higher (specific) SFRs, such as found in our zCOSMOS sample at z ≈ 0.7, agree with the predictions of a non-evolving Z(M, SFR).