GALAXY DOWNSIZING AND THE REDSHIFT EVOLUTION OF OXYGEN AND NITROGEN ABUNDANCES: ORIGIN OF THE SCATTER IN THE N/H–O/H DIAGRAM

The oxygen and nitrogen abundance evolutions with redshift of emission-line galaxies in the Sloan Digital Sky Survey are considered for four intervals of galaxy stellar masses, ranging from 1011.3 M☉ to 1010.2 M☉. We have measured their line fluxes and derived the O and N abundances using recent calibrations. The evolution of O and N abundances with redshift clearly shows the galaxy downsizing effect, where enrichment (and hence star formation) ceases in high-mass galaxies at earlier times and shifts to lower-mass galaxies at later epochs. The origin of the scatter in the N/H–O/H diagram has been examined. The most massive galaxies, where O and N enrichment and star formation have already stopped, occupy a narrow band in the N/H–O/H diagram, defining an upper envelope. The less massive galaxies which are still undergoing star formation at the current epoch are shifted downward, toward lower N/H values in the N/H–O/H diagram. This downward shift is caused by the time delay between N and O enrichment. This time delay together with the different star formation histories in galaxies is responsible for the large scatter in the N/H–O/H diagram.