On the use of the index N2 to derive the metallicity in metal-poor galaxie

TL;DR

This study evaluates the reliability of the N2 index for estimating metallicity in metal-poor galaxies, revealing it often overestimates true metallicity and is mainly useful for setting upper limits.

Contribution

The paper provides a critical assessment of N2-based metallicity calibrations in low-metallicity galaxies, highlighting their limitations and offering an explicit upper limit expression.

Findings

N2 index tends to overestimate metallicity in metal-poor galaxies.

Existing calibrations are only reliable for upper limit estimates.

Scatter in N2-metallicity relation is due to HII region evolution and N/O variations.

Abstract

The N2 index ([N II]6584/Ha) is used to determine emission line galaxy metallicities at all redshifts, including high redshift, where galaxies tend to be metal-poor. The initial aim of the work was to improve the calibrations used to infer oxygen abundance from N2 employing updated low-metallicity galaxy databases. We compare N2 and the metallicity determined using the direct method for the set of extremely metal-poor galaxies compiled by Morales-Luis et al. (2011). To our surprise, the oxygen abundance presents a tendency to be constant with N2, with a very large scatter. Consequently, we find that the existing N2 calibrators overstimate the oxygen abundance for most low metallicity galaxies, and then they can be used only to set upper limits to the true metallicity in low-metallicity galaxies. An explicit expression for this limit is given. In addition, we try to explain the observed scatter using photoionization models. It is mostly due to the different evolutionary state of the HII regions producing the emission lines, but it also arises due to differences of N/O among the galaxies.