The impact of the nitrogen-to-oxygen ratio on ionized nebulae diagnostics based on [NII] emission lines

TL;DR

This study examines how the nitrogen-to-oxygen ratio influences nebular diagnostics and metallicity estimates, revealing biases in common methods and the importance of N/O variations in galaxy classification and chemical evolution.

Contribution

It provides a detailed analysis of N/O effects on metallicity calibrators and diagnostic diagrams, highlighting biases and proposing more accurate interpretations.

Findings

N/O ratio causes underestimation of metallicity at low N/O.

High N/O ratios can mimic AGN-like signatures in diagnostic diagrams.

N/O correlates with stellar mass, with a plateau at high mass.

Abstract

We study the relation between nitrogen and oxygen abundances as a function of metallicity for a sample of emission-line objects for which a direct measurement of the metallicity has been possible. This sample is representative of the very different conditions in ionization and chemical enrichement that we can find in the Universe. We first construct the N/O vs. O/H diagram and we discuss its large dispersion at all metallicity regimes. Using the same sample and a large grid of photoionization models covering very different values of the N/O ratio, we then study the most widely used strong-line calibrators of metallicity based on [NII] emission lines, such as N2 and O3N2. We demonstrate that these parameters underestimate the metallicity at low N/O ratios and viceversa. We investigate also the effect of the N/O ratio on different diagnostic diagrams used to discriminate narrow-line AGNs from star forming regions, such as the [OIII]/Hbeta vs. [NII}]/Halpha, and we show that a large fraction of the galaxies catalogued as composite in this diagram can be, in fact, star forming galaxies with a high value of the N/O ratio. Finally, using strong-line methods sensitive to the N/O abundance ratio, like N2O2 and N2S2, we investigate the relation between this ratio and the stellar mass for the galaxies of the SDSS. We find, as in the case of the mass-metallicity relation, a correlation between these two quantities and a flattening of the relation for the most massive galaxies, which could be a consequence of the enhancement of the dispersion of N/O in the high metallicity regime.