Outliers from the Mass--Metallicity Relation II: A Sample of Massive Metal-Poor Galaxies from SDSS

TL;DR

This paper identifies 42 massive, star-forming galaxies with unusually low metallicities, likely due to gas inflow from interactions, impacting how galaxy metallicities are inferred from luminosity.

Contribution

It presents a new sample of massive, low-metallicity outliers and links their properties to gas inflow caused by tidal interactions.

Findings

Galaxies are extremely blue with high star formation rates.

Gas-phase oxygen abundances are diluted by low-metallicity gas from outer regions.

Implications for metallicity estimates in gamma-ray burst host galaxies.

Abstract

We present a sample of 42 high-mass low-metallicity outliers from the mass--metallicity relation of star-forming galaxies. These galaxies have stellar masses that span log(M_*/M_sun) ~9.4 to 11.1 and are offset from the mass--metallicity relation by -0.3 to -0.85 dex in 12+log(O/H). In general, they are extremely blue, have high star formation rates for their masses, and are morphologically disturbed. Tidal interactions are expected to induce large-scale gas inflow to the galaxies' central regions, and we find that these galaxies' gas-phase oxygen abundances are consistent with large quantities of low-metallicity gas from large galactocentric radii diluting the central metal-rich gas. We conclude with implications for deducing gas-phase metallicities of individual galaxies based solely on their luminosities, specifically in the case of long gamma-ray burst host galaxies.