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 ≤ z ≤ 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 starforming galaxies. For masses below 109 Mo, 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 Mo < M∗ < 1011 Mo: 12 + log (O/H) = (5.83 ± 0.19) +(0.30 ± 0.02) log (M∗/Mo). At 109 Mo < M∗ < 1010.5 Mo, 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 Mo. 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 Mo decreases. Our result implies that either the scatter of the baryonic accretion rate (δṀ? ) or the scatter of the M∗-Mhalo relation (δSHMR) 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 δṀ and δSHMR remain unchanged from z = 0.7 to z = 0