The mass-metallicity relation of zCOSMOS galaxies at z ~ 0.7, its
  dependence on SFR, and the existence of massive low-metallicity galaxies
  (Abridged) The knowledge of the number and of the physical nature of
low-metallicity massive galaxies is crucial for the determination and
interpretation of the mass-metallicity relation (MZR). Using VLT-ISAAC
near-infrared (NIR) spectroscopy of 39 zCOSMOS z~0.7 galaxies, we have measured
Halpha and [NII] emission line fluxes for galaxies with [OII], Hbeta and [OIII]
available from VIMOS optical spectroscopy. The NIR spectroscopy enables us to
break the degeneracy of the R23 method to derive unambiguously O/H gas
metallicities, and also SFRs from extinction corrected Halpha. Using, as a
benchmark, the position in the D4000 vs. [OIII]/Hbeta diagram of galaxies with
reliable O/Hs from NIR spectroscopy, we were able to break the lower/upper
branch R23 degeneracy of additional 900 zCOSMOS z~0.7 galaxies. Additionally,
the Halpha-based SFR measurements were used to find the best SFR calibration
based on [OII] for the zCOSMOS z~0.7 galaxies without Halpha measurements. We
find a fraction of 19% of lower mass 9.5<logM/Msun<10.3 zCOSMOS galaxies which
shows a larger evolution of the MZR relation, compared to higher mass galaxies,
being more metal poor at a given mass by a factor of 2-3 compared to SDSS. This
indicates that the low-mass MZR slope is getting steeper at z~0.7 compared to
local 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 SFR influences the MZR at these
redshifts. The comparison of the measured metallicities for the zCOSMOS sample
with the values expected for a non-evolving fundamental metallicity relation
(FMR) shows broadly agreement, and reveals that also galaxies with lower
metallicities and typically higher (specific) SFRs, as found in our zCOSMOS
sample at z~0.7, are in agreement with the predictions of a non-evolving
Z(M,SFR).
