The cosmic evolution of oxygen and nitrogen abundances in star-forming galaxies over the last 10 Gyrs
The chemical evolution of galaxies on a cosmological timescale is still a matter of debate despite of 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 a 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 20k bright sample of the zCOSMOS survey and using for the first time the nitrogen-to-oxygen ratio as a star formation rate independent tracer of the gas phase chemical evolution of galaxies. Emission-line galaxies both in the SDSS and 20k zCOSMOS bright survey were used to study the evolution from the local Universe of the $MZR up to a redshift 1.32, and the relation between stellar mass and nitrogen-to-oxygen ratio (MNOR) up to a redshift 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 all over the redshift range. We confirm the trend to find lower metallicities in galaxies of a given stellar mass in a younger Universe. This trend is even observed taking into account possible selection effects 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 into account the slope of the O/H vs. N/O relation 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.