STELLAR MASS–GAS-PHASE METALLICITY RELATION AT 0.5 ≤ z ≤ 0.7: A POWER LAW WITH INCREASING SCATTER TOWARD THE LOW-MASS REGIME

Abstract

ABSTRACT 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 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 * < 10 11 M ⊙ : 12 + log ( O / H ) = ( 5.83 ± 0.19 ) + ( 0.30 ± 0.02 ) log ( M * / M ⊙ ) . At 109 M ⊙ < M * < 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 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 M ˙ and σ SHMR remain unchanged from z = 0.7 to z = 0.