Prospects for extending the Mass-Metallicity Relation to low mass at high redshift: a case study at z~1

TL;DR

This study demonstrates that using multiple emission line diagnostics, specifically the Dopita et al. method, improves metallicity estimates of low-mass, high-redshift galaxies compared to traditional N2 diagnostics, despite remaining uncertainties.

Contribution

It shows that combining [NII] and [SII] lines with advanced diagnostics enhances metallicity measurements at low mass and high redshift, addressing limitations of the N2 method.

Findings

Dopita et al. diagnostic reduces systematic uncertainty in metallicity estimates.

[NII] and [SII] lines can be detected at low S/N in faint galaxies.

Future studies require larger samples to account for stochastic variations.

Abstract

We report J-band MOSFIRE spectroscopy of a low-mass (log$(M_*/M_\odot)=8.62^{+0.10}_{-0.06}$) star-forming galaxy at $z=0.997$ showing the detection of [NII] and [SII] alongside a strong H$\alpha$ line. We derive a gas-phase metallicity of log$(\text{O}/\text{H})=7.99^{+0.13}_{-0.23}$, placing this object in a region of $M_* - Z$ space that is sparsely populated at this redshift. Furthermore, many existing metallicity measurements in this $M_* - z$ regime are derived from only [NII]/H$\alpha$ (N2), a diagnostic widely used in high-redshift metallicity studies despite the known strong degeneracy with the ionization parameter and resulting large systematic uncertainty. We demonstrate that even in a regime where [NII] and [SII] are at the detection limit and the measurement uncertainty associated with the [NII]/[SII] ratio is high (S/N~3), the more sophisticated Dopita et al. diagnostic provides an improved constraint compared to N2 by reducing the systematic uncertainty due to the ionization parameter. This approach does not, however, dispel uncertainty associated with stochastic or systematic variations in the nitrogen-to-oxygen abundance ratio. While this approach improves upon N2, future progress in extending metallicity studies into this low-mass regime will require larger samples to allow for stochastic variations, as well as careful consideration of the global trends among dwarf galaxies in all physical parameters, not just metallicity.