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

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 10^9 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 10^8 M_☉ < M_* < 10^(11)M_☉: 12 + log(O/H)=(5.83 ± 0.19)+(0.30 ± 0.02)log(M_*/M_☉). At 10^9 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 10^9 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_Ṁ) 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_Ṁ and σ_(SHMR) remain unchanged from z = 0.7 to z = 0.

Files

Additional details