The S2N2 metallicity calibrator and the abundance gradient of M 33

TL;DR

This paper introduces the S2N2 diagnostic diagram as a reliable method for estimating metallicities in HII regions, demonstrating its effectiveness for high-redshift galaxies and deriving an abundance gradient for M33.

Contribution

The paper presents a new diagnostic diagram (S2N2) for metallicity estimation in HII regions, validated both empirically and theoretically, and applies it to M33 to determine its abundance gradient.

Findings

S2N2 diagram provides single-valued metallicity estimates across a wide range.

Effective for high-redshift (z ~ 2) HII galaxy metallicity measurements.

Derived M33's metallicity gradient as -0.05 dex/kpc.

Abstract

We introduce the log(Ha/[SII]6717+6731) vs. log(Ha/[NII]6583) (S2N2) diagnostic diagram as metallicity and ionisation parameter indicator for HII regions in external galaxies. The location of HII regions in the S2N2 diagram was studied both empirically and theoretically. We found that, for a wide range of metallicities, the S2N2 diagram gives single valued results in the metallicity-ionisation parameter plane. We demonstrate that the S2N2 diagram is a powerful tool to estimate metallicities of high-redshift (z ~ 2) HII galaxies. Finally, we derive the metallicity for 76 HII regions in M33 from the S2N2 diagram and calculate an O/H abundance gradient for this galaxy of -0.05 (+-0.01) dex kpc^-1.