Stellar Mass-Gas-phase Metallicity Relation at 0.5 ≤ z ≤ 0.7: A Power Law with Increasing Scatter toward the Low-mass Regime

We present the stellar mass ({M}_*)-gas-phase metallicity relation (MZR) and its scatter at intermediate redshifts (0.5≤slant z≤slant 0.7) for 1381 field galaxies collected from deep spectroscopic surveys. The star formation rate (SFR) and color at a given {M}_* of this magnitude-limited (R≲ 24 AB) sample are representative of normal star-forming galaxies. For masses below 10⁹ {M}_⊙ , our sample of 237 galaxies is ∼10 times larger than those in previous studies beyond the local universe. This huge gain in sample size enables superior constraints on the MZR and its scatter in the low-mass regime. We find a power-law MZR at 10⁸ {M}_⊙ < {M}_* \lt {10}¹¹ {M}_⊙ : 12+{log}(O/H)=(5.83+/- 0.19) +(0.30+/- 0.02){log}({M}_*/{M}_⊙ ). At 10⁹ {M}_⊙ < {M}_* \lt {10}^10.5 {M}_⊙ , our MZR shows agreement with others measured at similar redshifts in the literature. Our power-law slope is, however, shallower than the extrapolation of the MZRs of others to masses below 10⁹ {M}_⊙ . The SFR dependence of the MZR in our sample is weaker than that found for local galaxies (known as the fundamental metallicity relation). Compared to a variety of theoretical models, the slope of our MZR for low-mass galaxies agrees well with predictions incorporating supernova energy-driven winds. Being robust against currently uncertain metallicity calibrations, the scatter of the MZR serves as a powerful diagnostic of the stochastic history of gas accretion, gas recycling, and star formation of low-mass galaxies. Our major result is that the scatter of our MZR increases as {M}_* decreases. Our result implies that either the scatter of the baryonic accretion rate ({σ }_\dot{M}) or the scatter of the {M}_*-{M}_{halo} relation ({σ }_{SHMR}) increases as {M}_* decreases. Moreover, our measure of scatter at z=0.7 appears consistent with that found for local galaxies. This lack of redshift evolution constrains models of galaxy evolution to have both {σ }_\dot{M} and {σ }_{SHMR} remain unchanged from z=0.7 to z = 0.