Galaxy downsizing and the redshift evolution of oxygen and nitrogen abundances: origin of the scatter in the N/H-O/H diagram

TL;DR

This study analyzes how oxygen and nitrogen abundances in galaxies evolve with redshift across different stellar masses, revealing galaxy downsizing and explaining the scatter in the N/H-O/H diagram due to enrichment delays and star formation histories.

Contribution

It provides new insights into the redshift evolution of element abundances in galaxies and the origin of scatter in their abundance diagrams, considering galaxy mass and star formation history.

Findings

Galaxy downsizing affects element enrichment timing.

Massive galaxies show narrow N/H - O/H relation.

Scatter caused by N and O enrichment delay.

Abstract

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 10^11.3 M_sun to 10^10.2 M_sun. 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 has 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 downwards, towards 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.