Zusammenfassung
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.
Nutzer