Metal-poor, Strongly Star-Forming Galaxies in the DEEP2 Survey: The Relationship between Stellar Mass, Temperature-based Metallicity, and Star Formation Rate

TL;DR

This study identifies 28 metal-poor galaxies at z~0.8 to examine the fundamental metallicity relation, revealing significant dispersion and a weak positive SFR-metallicity correlation, indicating non-equilibrium gas processes and stochastic star formation.

Contribution

First direct metallicity measurements of low-mass, high-SFR galaxies at z~0.8, testing the fundamental metallicity relation in this regime.

Findings

Galaxies are 1.05 dex above the mass-SFR relation at z~1.

Galaxies are 0.23 dex below the local mass-metallicity relation.

Weak positive correlation between SFR and metallicity at fixed stellar mass.

Abstract

We report on the discovery of 28 $z\approx0.8$ metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [OIII]$\lambda$4363 emission line, which provides a "direct" measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFRs, with stellar mass estimates from modeling the optical photometry. We find that these galaxies are $1.05\pm0.61$ dex above the z~1 stellar mass-SFR relation, and $0.23\pm0.23$ dex below the local mass-metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains (0.29 dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of $\approx100^{+310}_{-75}$ Myr that suggests stochastic star formation. Combining our sample with other z~1 metal-poor galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar mass) that is significant at 94.4% confidence. We interpret this positive correlation as recent star formation that has enriched the gas, but has not had time to drive the metal-enriched gas out with feedback mechanisms.