The Metal Abundance across Cosmic Time ($\mathcal{MACT}$) Survey II: Evolution of the Mass-Metallicity Relation over 8 Billion Years, using [OIII]$λ$4363Å-based Metallicities

Chun Ly, Matthew A. Malkan, Jane Rigby, Tohru Nagao

Published 2016-02-02Version 1

[Abridged] We present the results of MMT and Keck spectroscopy for a large sample of $0.1\leq z\leq1$ emission-line galaxies selected from our narrow-band imaging in the Subaru Deep Field. In total, we have measured the weak [OIII]$\lambda$4363 line for 164 galaxies (66 with at least 3$\sigma$ detections, and 98 with significant upper limits). The strength of this nebular emission line is set by the electron temperature ($T_e$) for the ionized gas in these galaxies. Since the gas temperature is regulated by the metal content, an inverse relationship exists between gas-phase oxygen abundance and [OIII]$\lambda$4363 line strength. Our $T_e$-based metallicity study is the first to span $\approx$8 Gyr of cosmic time and $\approx$3 dex in stellar mass for low-mass galaxies, $\log{(M_{star}/M_{sun})}\approx6.0-9.0$. Combined with extensive multi-wavelength photometry, we investigate the evolution of the stellar mass--gas metallicity relation, and its dependence on dust-corrected star formation rate. The latter is obtained from high signal-to-noise Balmer emission-line measurements. Our mass-metallicity relation is consistent with Andrews & Martini at $z\leq0.3$, and evolves toward lower abundances at a given stellar mass, $\propto(1+z)^{-2.32^{+0.53}_{-0.35}}$. We find that galaxies with lower metallicities have higher star formation rates at a given stellar mass and redshift, although the scatter is large ($\approx$0.3 dex) and the trend is weaker than seen in local studies. We also compare our mass--metallicity relation against predictions from high-resolution galaxy formation simulations and find good agreement with models that adopt energy and momentum stellar feedback. In addition, we have identified 16 extremely metal-poor galaxies with abundances less than a tenth of solar; our most metal-poor galaxy at $z\approx0.85$ has an oxygen abundance that is similar to I Zw 18.