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 Msun to 1010.2 Msun. 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.