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

¹ University of ViennaDepartment of Astrophysics, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: christian.maier@univie.ac.at
² Institute of Astronomy, ETH Zurich, 8093 Zurich, Switzerland
³ Institut de Recherche en Astrophysique et Planétologie, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
⁴ IRAP, Université de Toulouse, UPS-OMP, 31400 Toulouse, France
⁵ Instituto de Astrofísica de Andalucía, CSIC, Apartado de correos 3004, 18080 Granada, Spain
⁶ INAF−Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ CEA-Saclay, Orme des Merisiers, Bat. 709, 91191 Gif-sur-Yvette, France

Received: 27 October 2014
Accepted: 24 February 2015

Abstract

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).