The cosmic evolution of oxygen and nitrogen abundances in star-forming galaxies over the past 10 Gyr

¹ Institut de Recherche en Astrophysique et Planétologie, CNRS, 14 avenue Édouard Belin, 31400 Toulouse, France
² IRAP, Université de Toulouse, UPS-OMP, Toulouse, France
³ Instituto de Astrofísica de Andalucía, CSIC, Apartado de correos 3004, 18080 Granada, Spain
e-mail: epm@iaa.es
⁴ Institute of Astronomy, ETH Zurich, 8093 Zürich, Switzerland
⁵ Laboratoire d’Astrophysique de Marseille, CNRS-Université d’Aix-Marseille, 38 rue Frédéric Joliot Curie, 13388 Marseille, France
⁶ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b. Muenchen, Germany
⁷ Dipartimento di Astronomia, Università di Padova, vicolo Osservatorio 3, 35122 Padova, Italy
⁸ INAF – IASF Milano, via Bassini 15, 20133 Milano, Italy
⁹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
¹⁰ Max-Planck-Institut für extraterrestrische Physik, 84571 Garching b. Muenchen, 85748, Germany
¹¹ SUPA Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh, EH9 3HJ, UK
¹² INAF – Osservatorio Astronomico di Trieste, via Tiepolo, 11, 34143 Trieste, Italy
¹³ INAF – Osservatorio Astronomico di Brera, via Brera, 28, 20159 Milano, Italy
¹⁴ MPA – Max Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
¹⁵ University of Vienna, Department of Astronomy, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
¹⁶ Universitá degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
¹⁷ Instituto de Astrofísica de Canarias, vía Lactea s/n, 38200 La Laguna, Tenerife, Spain
¹⁸ IPMU, Institute for the Physics and Mathematics of the Universe, 5-1-5 Kashiwanoha, 277-8583 Kashiwa, Japan
¹⁹ INAF – IASFBO, via P. Gobetti 101, 40129 Bologna, Italy
²⁰ Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands

Received: 20 July 2012
Accepted: 28 September 2012

Abstract

Aims. The chemical evolution of galaxies on a cosmological timescale is still a matter of debate despite the increasing number of available data provided by spectroscopic surveys of star-forming galaxies at different redshifts. The fundamental relations involving metallicity, such as the mass − metallicity relation (MZR) or the fundamental metallicity relation, give controversial results about the reality of evolution of the chemical content of galaxies at a given stellar mass. In this work we shed some light on this issue using the completeness reached by the 20 k bright sample of the zCOSMOS survey and using for the first time the nitrogen-to-oxygen ratio (N/O) as a tracer of the gas phase chemical evolution of galaxies that is independent of the star formation rate.

Methods. Emission-line galaxies both in the SDSS and 20 k zCOSMOS bright survey were used to study the evolution from the local Universe of the MZR up to a redshift of  ~1.32, and the relation between stellar mass and N/O (MNOR) up to a redshift of  ~0.42 using the N2S2 parameter. All the physical properties derived from stellar continuum and gas emission-lines, including stellar mass, star formation rates, metallicity and N/O, were calculated in a self-consistent way over the full redshift range.

Results. We confirm the trend to find lower metallicities in galaxies of a given stellar mass in a younger Universe. This trend is even observed when taking possible effects into account that are due to the observed larger median star formation rates for galaxies at higher redshifts. We also find a significant evolution of the MNOR up to z ~ 0.4. Taking the slope of the O/H vs. N/O relation into account for the secondary-nitrogen production regime, the observed evolution of the MNOR is consistent with the trends found for both the MZR and its equivalent relation using new expressions to reduce its dependence on star formation rate.